A rootkit is software that enables continued privileged access

Privileged access

In the fields of epistemology and philosophy of mind it is said the first person has privileged access to his own thoughts. This implies the subject has access to his own thoughts in such a way that the third person do not...

 to a computer while actively hiding its presence from administrators by subverting standard operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 functionality or other applications. The term rootkit is a concatenation

Concatenation

In computer programming, string concatenation is the operation of joining two character strings end-to-end. For example, the strings "snow" and "ball" may be concatenated to give "snowball"...

 of "root"

Superuser

On many computer operating systems, the superuser is a special user account used for system administration. Depending on the operating system, the actual name of this account might be: root, administrator or supervisor....

 (the traditional name of the privileged account on Unix

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 operating systems) and the word "kit" (which refers to the software components that implement the tool).