Quick Definition: The conscious and unconscious behaviors of a woman that convey she is interested in meeting the PUA.
Approach Invitations are subtle signals that women give when they are interested in, attracted to, or curious about a particular person and want that person to approach them. Due to social programming, most women must do this without being overly blatant. Common examples of AIs include a woman looking at the PUA, then looking down at the floor, then looking back at him again; proximity (standing near the PUA in the hopes that he will strike up a conversation with her); or simply smiling at the PUA. AIs, in other words, are non-verbal IOIs that a woman gives before a PUA enters a set.
AIs can help a PUA focus on warmer sets rather than wasting time doing cold approaches on women who are not interested. When given warm AIs, the PUA’s game will become significantly more efficient. It is important not to fall into the trap of AFC reversion, which some guys do if they get overly aggressive AIs such as a wink or DDBL (I.e. Cougars). A woman’s attraction during AI is still based more on curiosity than “I want to have sex with you right now.”
Notice how nightclub promotions always use a “come hither” look from their photography of the girls at the parties: