1. A computer user who illegally visits networked computers to look around and/or cause harm. A criminal. From Glossary of Distance Education and Internet Terminology

2. A specific type of hacker who decrypts passwords or breaks software copy protection schemes (creating "crackz"). Also, a generic name used by some to refer to all "malicious" hackers. Controversy: This work is extremely controversial. See the word hacker for a discussion about the way that "cracker" is used in the computer enthusiast community vs. the security community. From Hacking-Lexicon