A Practical Glossary of Mobile OS and App Terms
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From APK to sandbox to WebKit — a concise reference guide to the technical terms you'll encounter when reading about smartphones.
Why This Glossary Exists
Smartphone reviews, OS release notes, and app store descriptions are dense with technical shorthand. Terms like APK, WebKit, and sandbox appear constantly — but rarely with explanation. This reference covers the vocabulary you'll encounter most when reading about Android, iOS, and the software ecosystems built on top of them.
For a broader picture of what these components do together, see what a mobile operating system actually does. If you're specifically trying to understand how apps function at a conceptual level, key concepts about how apps work is a useful companion.
Kernel
The lowest-level component of an operating system that manages communication between software and hardware. On Android, this is a modified Linux kernel; iOS uses its own XNU kernel derived from BSD Unix.
HAL (Hardware Abstraction Layer)
A software interface that allows the OS to communicate with hardware components without needing to know their specific implementation details. It lets Android run on phones from many different manufacturers.
Sandbox
An isolated execution environment that prevents an app from accessing the data or processes of other apps. Both iOS and Android enforce sandboxing at the OS level as a core security mechanism.
API Level
An integer that identifies a specific version of the Android platform's public API. Apps declare a minimum and target API level; this determines which OS features they can use and which devices they can run on.
ART (Android Runtime)
The managed runtime environment that executes Android app code. ART replaced the older Dalvik runtime and compiles app bytecode ahead-of-time for improved performance.
Sideloading
Installing an app from outside an official platform store — such as loading an APK directly on Android. The degree to which platforms allow or restrict sideloading varies and carries security tradeoffs.
OTA Update
Over-the-Air update — a software or firmware update delivered wirelessly to a device without requiring a physical connection to a computer or a manual file installation.
Deep Link
A URL or intent that opens a specific screen or action within an installed app rather than a web browser. App-to-app navigation on both Android and iOS relies heavily on deep linking standards.
Entitlement
On iOS, a key-value pair embedded in an app's code signature that grants access to specific capabilities — such as push notifications, HealthKit, or iCloud. Apps must be explicitly approved for each entitlement.
Bytecode
An intermediate, platform-neutral code format that an app compiler produces. On Android, bytecode is executed by ART; this abstraction layer is what allows the same app to run across different processor architectures.
Foreground Service
An Android component that performs an operation the user is actively aware of — such as music playback or navigation — and displays a persistent notification. It receives higher OS priority than background processes.
Extensions (iOS)
Small, sandboxed processes that allow an app to provide functionality to other parts of the OS — such as a photo editing tool appearing in the Photos app, or a keyboard available system-wide.
Core OS and Runtime Terms
These terms describe the foundational layers of a mobile operating system — from how the hardware is addressed to how apps are executed.
| Android kernel base | Modified Linux kernel (Android Open Source Project documentation) |
| iOS kernel base | XNU (derived from BSD Unix) (Apple developer documentation) |
| Android runtime (current) | ART (Android Runtime) (Replaced Dalvik as of Android 5.0 (2014)) |
| iOS mandatory browser engine | WebKit (all browsers on iOS) (Apple App Store Review Guidelines) |
| Runtime permissions introduced | Android 6.0 (Marshmallow), 2015 (Android developer documentation) |
| Android push notification service | Firebase Cloud Messaging (FCM) (Google Firebase documentation) |
Understanding these distinctions matters when evaluating OS update support. A phone receiving only security patches is not the same as one receiving full OS version upgrades — the former patches known vulnerabilities while the latter adds new runtime capabilities and API levels that apps may rely on.
For a security-focused look at how the OS isolates apps from each other, sandboxing explained goes deeper on that topic.
App Distribution and Installation Terms
How an app reaches your device — and what format it arrives in — is governed by a distinct set of platform-specific conventions.
APK (Android Package Kit): The file format Android uses to package and distribute apps. While Google Play handles APK delivery automatically, users can also install APKs manually from outside the Play Store — a practice called sideloading. Sideloading bypasses Google's vetting process and carries security considerations covered in common cyber threat terms.
IPA (iOS App Archive): Apple's equivalent package format for iOS apps. Unlike APKs, IPAs cannot be sideloaded on standard consumer devices without developer tools or specific entitlements — a deliberate platform constraint.
App Bundle (AAB): A newer Android format that lets Google Play generate optimized APKs tailored to each device's screen density and CPU architecture, reducing download size.
TestFlight: Apple's platform for distributing pre-release iOS apps to testers before App Store submission. Developers use it during beta cycles.
Knowing these formats helps when a review references sideloaded builds, beta APKs, or TestFlight access — all of which describe apps outside the standard distribution pipeline. For related vocabulary about what app permission requests mean, a dedicated reference breaks those down clearly.
Rendering, Permissions, and Background Processes
WebKit: The browser engine originally developed by Apple and used as the mandatory rendering engine for all browsers on iOS — including Chrome and Firefox on that platform. Android allows alternative engines, which is why browser performance can differ meaningfully between platforms.
Runtime Permissions: A model (adopted by Android 6.0 and later mirrored in iOS) where apps must request access to sensitive resources — camera, microphone, location — at the moment of use rather than at installation. This gives users finer-grained control.
Background App Refresh: A mechanism that allows apps to fetch updated content while not actively in use. Both iOS and Android implement aggressive throttling of this behavior to preserve battery life, but the specific rules differ by platform and user setting.
Push Notification Service: Apps don't maintain constant server connections; instead, they rely on platform-managed services — Apple Push Notification service (APNs) on iOS and Firebase Cloud Messaging (FCM) on Android — to deliver notifications efficiently.
If AI-specific terms are appearing alongside these in device specs, a reference guide to AI terms in smartphone specs covers that vocabulary separately.
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Smartphones Editorial Team
Smartphones Editorial Team is the collective byline for our editorial team and contributor network. Articles published under this byline or an editorial pen name are researched, written, and reviewed according to our editorial standards for clarity, consistency, and independence before publication.
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