Final answer:
A Rootkit is malware that changes system functionality at the OS level for a malicious purpose by gaining administrative access after installation, hiding its presence and enabling privileged access.
Step-by-step explanation:
The threat that changes system functionality at the operating system level for a malicious purpose by gaining administrative access after being installed by a user is known as a Rootkit. Unlike other forms of malware, a rootkit is designed to hide the existence of certain processes or programs from normal methods of detection and enable continued privileged access to a computer. Rootkits typically modify the host operating system's software, whether that's the kernel itself or less critical user-mode applications. Rootkits are particularly insidious and can enable the attacker to gain control over an entire system without the user's knowledge.