Android is a mobile operating system developed by Google, based on the Linux kernel and designed primarily for touchscreen mobile devices such as smartphones and tablets.

Android's user interface is mainly based on direct manipulation, using touch gestures that loosely correspond to real-world actions, such as swiping, tapping and pinching, to manipulate on-screen objects, along with a virtual keyboard for text input.

In addition to touchscreen devices, Google has further developed Android TV for televisions, Android Auto for cars, and Android Wear for wrist watches, each with a specialized user interface.

Variants of Android are also used on notebooks, game consoles, digital cameras, and other electronics.

Android Package Kit (APK) is the package file format used by the Android operating system for distribution and installation of mobile apps and middleware.

A watermark is an identifying image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations in the paper.

A mobile application software or mobile app is an application software designed to run on mobile devices such as smartphones and tablet computers.

Most such devices are sold with several apps bundled as pre-installed software, such as a web browser, email client, calendar, mapping program, and an app for buying music or other media or more apps.

Some pre-installed apps can be removed by an ordinary uninstall process, thus leaving more storage space for desired ones.

Where the software does not allow this, some devices can be rooted to eliminate the undesired apps.

In computer networks, to download is to receive data from a remote system, typically a server such as a web server, an FTP server, an email server, or other similar systems.

This contrasts with uploading, where data is sent to a remote server.