1) Verb: To instantaneously and uncontrollably jizz in one's pants upon the thought, mention, sight, presence, brief musings, discussion, pondering, or stalking of a person, place where person hangs out, or thing the person likes, loves, or maybe looked sideways at.... or maybe he or she just thinks about it... maybe... you will find out.... you must find out!
2) Noun: An act of instantaneously, and uncontrollably jizzing in one's pants as a result of the aforementioned reasons.
Girl 1: So I heard you saw Terry Hatcher yesterday.. She's so pretty!
Girl 2: OMG yeah I know, my ex-boyfriend always used to tell me how she was such an instagasm (it's part of why we broke up, can't have my man chasing another woman).
Girl 1: What's an instagasm?
Girl 2: Look it up on urban dictionary, duh!