A security hacker is someone who explores methods for breaching defenses and exploiting weaknesses in a computer system or network. Hackers may be motivated by a multitude of reasons, such as profit, protest, information gathering, challenge, recreation, or evaluation of a system weaknesses to assist in formulating defenses against potential hackers.

Longstanding controversy surrounds the meaning of the term "hacker." In this controversy, computer programmers reclaim the term hacker, arguing that it refers simply to someone with an advanced understanding of computers and computer networks, and that cracker is the more appropriate term for those who break into computers, whether computer criminals (black hats) or computer security experts (white hats). A 2014 article noted that "the black-hat meaning still prevails among the general public". The subculture that has evolved around hackers is often referred to as the "computer underground".