A girl who is not a girlfriend. Shares interests with, hangs with, and (optionally) sleeps with a man she is interested in. Inevitably, she becomes a 'cool girl' and becomes ineligible for girlfriend status. Is often beleived to be lesbian on first glance.
Guy 1: Sam likes Halo? You serious? Damn, you're lucky to have a girlfriend like that.
Guy 2: She's not my girlfriend. She's just a cool girl.