1. Sexual intercourse between people who are too closely related to marry (as between a parent and a child).
2. Sexual relations between people who are so closely related that their marriage is illegal or forbidden by custom.

The statutory crime of sexual relations with such a near relative.

3. Etymology: from about 1225, "the crime of sexual intercourse between near kindred", from Latin incestum, "unchastity, not chaste, impure"; also specifically, "sex between close relatives"; from in-, "not" + castus, "pure".

In early use also with reference to "sexual relations" or "marriage with someone under a vow of chastity; sometimes distinguished as spiritual incest".