Sidechick

[Little_Girl]

the other woman; also known as the [mistress]; a female that is neither a males wife or girlfriend who has relations with the male while he is in another relationship

[Donald Trump]

A position allocated to a girl which is neither a wifey or a girlfriend but a side dish like nandos rice

[Mark Zuckerberg]

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