Saily eSIM glossary
Saily’s eSIM glossary defines all the key terms you might need to know about eSIM technology, mobile data, and telecommunications. Not sure what a specific term means or how an eSIM feature works? You’re in the right place. Our glossary is researched and written by eSIM experts and is regularly updated.
Last updated September 14, 2026
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Fifth-generation mobile
Commonly known as 5G, the fifth generation of communications standards is gradually replacing 4G around the world. Using 5G, you can enjoy faster speeds (with average downloaded speeds of 1-10 Gbps) and lower latency (1ms under ideal conditions, compared with 4G’s 45ms). 5G can use higher radio frequencies, wider channel bandwidths, and advanced antenna techniques like massive MIMO, so it supports a broader range of use cases than previous cellular generations.
A
Airplane mode is a setting on your device that turns off all wireless transmissions, including cellular, Wi-Fi, and Bluetooth. It was originally required during flights to prevent possible interference with aircraft systems. When airplane mode is on, your eSIM profile stays installed but becomes inactive until cellular data is turned back on. On most devices, you can still manually enable Wi-Fi and Bluetooth while airplane mode is on.
Access point name
APN, or access point name, is the collection of settings your phone uses to connect to your carrier’s mobile network. It includes details like the network address your phone uses to reach the internet, the settings needed for things like MMS messages, and other configuration data that helps your mobile data work properly. In most cases, your carrier automatically provides and configures the correct APN settings for you.
C
Mobile network operator / Network
A carrier is a telecom service provider — the company you pay for mobile data. Carriers buy radio spectrum licenses at government auctions and either own or license physical infrastructure (like cell towers) so they can offer wireless services to their customers. These companies sell directly to individual users but can also provide wholesale connectivity to MVNOs. Examples of carriers in the US include AT&T and Verizon.
A coverage map is a visual representation of the areas where a mobile network operator or eSIM provider offers service. It helps you see where you’ll have mobile signal and data access before you travel or buy a plan. Coverage maps are especially useful if you’re traveling because they let you check which countries and regions are supported before you travel.
D
Download speed is the rate at which data moves from the internet to your device, usually measured in megabits per second (Mbps). It determines how quickly you load web pages, stream videos, and use online services like apps. LTE networks can give you download speeds of up to 100 Mbps, while top-performing 4G LTE networks can deliver peak download speeds of up to 100 Mbps and upload speeds of up to 30 Mbps. 5G networks are even faster.
E
eSIM identifier / embedded identity document
An EID (embedded identity document, or eSIM identifier) is a unique 32-digit number permanently assigned to your device’s eSIM chip. It’s like a serial number for the embedded SIM hardware. EIDs are used in remote provisioning and eUICC management. You’ll need an EID if you’re manually activating an eSIM or contacting the eSIM carrier for support, and it plays a big part in managing eSIM profiles and troubleshooting connectivity problems.
Embedded subscriber identity module
An eSIM is a SIM that’s built directly into your device instead of being a removable physical card. It lets you activate and switch mobile plans digitally without needing to insert or replace a SIM card. Unlike a traditional SIM, an eSIM can store and manage multiple carrier profiles, making it easier to change networks or add travel plans when you need them.
Embedded universal integrated circuit card
eUICC (embedded universal integrated circuit card) is the hardware inside a device that makes eSIM connectivity possible. It’s a built-in version of a SIM card that’s soldered directly onto the device’s circuit board, usually as a small chip that can’t be tampered with or easily removed. An eUICC securely stores and manages multiple carrier profiles, allowing you to download and switch mobile subscriptions remotely without needing to physically replace a SIM card.
G
Gigabyte
A gigabyte (GB) is a unit of digital data equal to 1024 megabytes (MB), or roughly 1 billion bytes. Mobile data plans are usually sold in GB amounts like 1 GB, 5 GB, or 10 GB. As a rough guide, 1 GB can support about 8 hours of social media browsing, around two hours of SD video streaming, or the loading of more than 200 web pages.
H
Mobile hotspot
A hotspot is a feature (or the physical location on a device) that allows your device to provide wireless internet access. On a smartphone, a mobile hotspot lets you share your device’s data connection with other hardware (like laptops or tablets) via Wi-Fi — a process called tethering. When you buy an eSIM plan, check whether tethering and hotspot sharing is included, because some plans restrict it or charge extra for this feature.
I
Integrated circuit card identifier
An ICCID is a unique number of 19 or 20 digits assigned to a SIM card to identify it on a network. Every SIM or eSIM profile has its own ICCID that helps carriers recognize and manage the specific SIM being used. Once an eSIM profile is installed on an eUICC, it works just like a physical SIM, using its own ICCID. You can usually find the ICCID in your device settings when a SIM is installed.
International mobile subscriber identity
An IMSI (international mobile subscriber identity) is a unique number, usually 15 digits long, that identifies a subscriber on a mobile network. It’s assigned to a SIM card by the telecom provider and is tied to the SIM rather than the device itself. The IMSI can also indicate the subscriber’s country and mobile network. With an eSIM, the IMSI can be updated or replaced through software-based profile downloads, which makes it easier to switch between networks and eliminates the need for a physical SIM.
Internet Protocol address
An Internet Protocol address (or IP address) is a unique numerical label assigned to each device connected to a computer network. The device can be identified and located by its IP address, allowing it to send and receive data. Mobile networks assign IP addresses whenever new devices connect. If you use a travel eSIM, your device is assigned a local IP address from the eSIM app’s local partner network.
L
Latency is the time delay between sending a request from a device and receiving a response over a network, measured in milliseconds (ms). A typical 4G network has a latency of approximately 45ms compared to 5G latency, which can be as low as 1ms (under ideal circumstances). Low latency is essential for real-time applications like video calls, online gaming, and self-driving car communications.
Long Term Evolution
LTE is a 4G wireless broadband standard that provides faster speeds and greater network capacity for mobile users. It can (under ideal circumstances) deliver peak download speeds of up to 100 Mbps and upload speeds of up to 30 Mbps. LTE is the most widely used 4G technology worldwide and is the backbone of many mobile and eSIM data plans (though 5G is becoming more widespread).
M
Megabyte
MB, or megabyte, is a unit of digital data equal to 1,000 kilobytes (KB), or approximately 1 million bytes. For context, 1 GB contains 1024 MB. Mobile data usage is often measured in MB for smaller amounts of activity, like browsing the web or sending emails. For example, sending an email with a photo attachment typically uses around 1-3 MB of data, while loading an average web page can use 2-3 MB.
Mobile network operator
An MNO (mobile network operator) is a carrier that owns and runs mobile network infrastructure, including cell towers and radio spectrum licenses, and provides wireless service directly to customers. It can also sell network access in bulk to MVNOs, companies that then resell mobile services under their own brands. Examples of MNOs include AT&T, Verizon, T-Mobile, and Vodafone.
Mobile data is internet access provided through a cellular network like 3G, 4G LTE, or 5G. It lets you browse the web, use apps, stream videos, and stay connected across multiple locations. Your mobile data speed is the rate at which information is sent and received, usually measured in kilobits per second (kbps) or megabits per second (Mbps).
Mobile virtual network operator
An MVNO (mobile virtual network operator) is a mobile carrier that doesn’t own its own wireless network infrastructure. Instead, it leases infrastructure access from larger network carriers (MNOs). MVNOs “piggyback” on these existing networks to offer coverage through their own mobile plans. Many eSIM providers work as MVNOs, offering lower-cost plans by using established networks.
N
Network congestion happens when too many users are connected to the same tower at once, reducing the available bandwidth and slowing down data speeds for everyone. It’s very common in busy cities and tourist hubs, where both locals and visitors might all be using the same network resources. Even if an eSIM provider offers strong coverage, speeds can still drop when a local tower is overloaded and everyone is competing for capacity.
P
Personal identification number
A PIN (personal identification number) is a four-digit or longer number associated with a SIM card. You type in the password whenever you start your device. This security feature can be turned off in your phone settings. If you input an incorrect PIN three times, the SIM card locks and can only be accessed using a PUK (personal unblocking key) number, issued by your network provider.
S
Physical SIM / Subscriber identity module
A SIM card is a removable smart card inside cellular devices (like smartphones) that identifies and authenticates a user on a mobile network. The card stores information like your customer ID and phone number, which lets you move your mobile service between different GSM-compatible phones. It can also store information like contacts and SMS messages.
V
Voice over Internet Protocol
Voice over Internet Protocol (VoIP) is a technology that allows voice calls and multimedia communication to be delivered over Internet Protocol (IP) networks like the internet. Popular apps like WhatsApp, FaceTime, Skype, and Google Meet use either mobile data or Wi-Fi instead of traditional cellular voice networks to make calls and send messages.
Voice over LTE
The term VoLTE (voice over LTE) refers to voice calls made over a 4G LTE network. This standard gives you clearer call quality than with older networks like 3G (often called HD voice) and lets you use voice and mobile data at the same time on LTE. When you’re using a travel eSIM alongside your home SIM, it’s worth checking whether the local partner network supports VoLTE.
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