The first person you have romantic feelings for. Usually the boy/girl-next-door, that popular douche, your
elder sibling's really hot friend and in some cases, your
Chemistry teacher.
Boy: Hey, why are you hiding from Mr Magnesium
Oxide?
Girl:
SHHHHH OMg, he is my first crush!
(See that OMg there *wink**wink* Gah, I know it's
MgO)