Quick Definition: When a girl signals an action that discourages your approach (the opposite of AI).
Full Definition:
Anti-AI can take several forms. It can occur on a more neutral level when a target attempts to be unapproachable to the pickup artist, such as being surrounded by her friends all night and deliberately walks away or avoids eye contact when near your proximity. It can also refer to negative actions that discourage your approach, such as a cold, disgusted, or unemotional look or evasive body language.

This is an exaggerated instance of an bad reaction. Learn to read subtle reactions from girls
Anti-AI makes for a more challenging approach than opening a set that is clearly interested in you, but this does not mean that a set in unapproachable. Also, sometimes a girl may seems uninterested with her body language and facial expression and yet be in a really good mood when you say hello. Other times, girls who appear to be in high spirits can treat you very poorly. While body language reading patterns can be used, one is never really sure. You never know someone until you know them, and the only way to do this is to say hi.
Usage:
I am getting a lot of Anti-AI signals. Are the girls unfriendly here or is it this fugly hat I am wearing?
Related Terms: IOI, Approach Invitation, IOD, Bitch Shield, Proximity