Rooting is the act of gaining root access to an android mobile phone OS, in-order to have higher administrative access, to alter the operating system code of the android device.
A rooted android device is a mobile phone running android operating system, which has been altered or jailbroken, to gain higher administrative controls, such as; installation of unapproved apps, deletion of pre-installed OS-bundled apps (i.e. system apps), updating of OS, installation of firmware, and underclocking or overclocking of the processor.