The term refers to a specific version of the Android operating system, version 5.1 (Lollipop), packaged as an Android Package Kit file. This file format is used for distributing and installing mobile applications on the Android operating system. As an example, a user seeking to install an application compatible with a device running Android 5.1 may search for the appropriate “android 5.1 apk” of that application.
The availability of application packages tailored for this operating system version is significant for users with older devices that cannot be upgraded to more recent Android versions. These packages ensure continued functionality and access to applications, preserving the usability of older hardware. Historically, this operating system version represented a key stage in the evolution of the Android platform, introducing features and improvements over its predecessors.