The person you are cheating on your boyfriend( or girlfriend) on

sidechick is known as the mistress; a female that is neither a male's wife or girlfriend who has relations with the male while he is in another relationship

A dusty, thirsty bird that one keeps around strictly for beating up the hot box. You usually have to gas her up by alluding to the possibility of a relationship in the near future, in order to get the buns. A throw away broad that an individual wears out like a boss, unbeknownst to said individual's main squeeze. Side-chicks are usually cool with their status as a slide-off due to the fact that they either have no shame or they're blinded by love. Not someone that you'd want to bone without a rubber. Guys can be side-chicks as well which could be equally, if not even more pathetic.

The kind of girls without a boyfriend who are often "the third wheel" in a relationship setting.
Usually these girls want to be with the guy whom they're associated with, but there was no agreement for them to go that step further.

The man whom the side chick is associated with is pretty indecisive about whom they want romantic relations with, but since they got hung up on several chicks, he might as well have his side chicks.

The girl a guy cheats on his girlfriend with because she's better in bed. Not always a bad thing to the girl and guy, just society.

Girls you have sex with on and off whilst you're looking for the one you want to go out with.

that bitch you keep on the side while you’re still dating someone