Definite is an adjective describing something that is known for certain. For example, there is no more definite way to get into trouble with a police officer than speeding in front of the police station with a broken taillight.
This word is from the Latin definitus meaning "defined" or "limited." Definite can also be used to mean something is very clearly planned or certainly defined. If you have definite plans for the evening to go to the theater and out for a nice dinner at a steak house, you're probably not going to end up playing miniature golf and eating fast food instead.