1. Generally speaking, any derivation or departure from the normal or usual course.
2. Sexually speaking, behavior deviating from what is considered normal sexual practices. In a more restrictive sense, any departure from genital-to-genital sexual intercourse between a man and a woman.
3. In psychology, an abnormal alteration in one's state of mind.
4. In genetics, a deviation from the normal structure or number of chromosomes in an organism.