1. v. To break into a system (compare cracker).
2. v. Action of removing the copy protection from a commercial program. People who write cracks consider themselves challenged by the copy protection measures. They will often do it as much to show that they are smarter than the developer who designed the copy protection scheme than to actually copy the program.
3. n. A program, instructions or patch used to remove the copy protection of a program or to uncripple features from a demo/time limited program.
4. An exploit.