An eyegasm is a sexual reaction when reading or seeing something that makes you sexually aroused in the brain. When seeing something that "turns you on", your eyes grow larger and you get interested. THAT is an eyegasm. They are mostly common in female teens and young adults.
/Sees a note from long lost crush about a date/
Person 1: Oh My, I just had an eyegasm!
Person 2: Dude, that's embarrassing!