What term is used to describe terminating a process that is not responding to user

input?

Kill or Killing the Process

Explanation:

When a program / process that is running on a computer ceases to respond to user input, the program is said to be "hanging." When you exit a program normally, this is usually referred to as just exiting or quitting the program. When a program is hanging, you usually have to "kill" the process.

This term is used because the cessation of the program execution is sudden and forceful.