copulation: normally refers to the first moment of initial entry of a penis into a vagina, mouth or anus.
the act of allowing to enter; admission
An intermeddling with the affairs of another, either on legal grounds or without authority.
the state of being allowed to enter; admittance
putting one thing into another; insertion
A session of sexual intercourse.
Used other than figuratively or idiomatically: see roll, in, the, hay.