'tis when a woman's nipple gets real claustrophobic and sticks its head out for a deep breath, while the woman does not notice at first. This is very common with actresses. Their nipples get nervous a lot, and need some air sometimes. A lot of the time, they stick their head out, and decide that they like all the attention so they linger longer than needed. This usually causes embarassment.
When Tara Reid was on the red carpet, she experienced a nipple slip. Luckily a friend nearby helped her out, for she was unaware that her nipple was getting more attention than her the entire time.