1) Verb. Mild romantic inclinations towards somebody. Most common in young teens although can happen to anybody. They are often non-sexual and can be very awkward for everyone involved. They are most common in females but can often occur in males too. The crush could be both somebody they know or a celebrity.
2) Noun. A person who is being crushed(1) on.
1) She has such a massive crush on Zac Efron. It was funny when she was younger but now it's just embarrasing.
2)She's making an extra effort with her hair today because somebody told her that her crush might be there.