A style, usually featuring well-developed women, which accentuates sexy with situations where the women are either innocent, unaware of their bodies, or confident.
Pin-ups are intended for informal display, i.e. meant to be "pinned-up" on a wall. Pin-up models may be glamour models, fashion models, or actors.
In comic books, a pin-up is simply a full-page piece of artwork, most often without dialogue, that showcases a character, group of characters, or significant event, published within an issue, rather than made available by itself as a poster.
From wikipedia