1. Literally, someone who bites their fingernails, especially due to nervousness or agitation. I was a nail-biter at a young age, and, try as I might, I've never managed to shake the habit completely.

2. By extension, a situation, especially a competitive one, whose outcome is particularly close or uncertain and marked by nervous apprehension. Did you catch the football match last Sunday? Man, what a nail-biter! With how close these two candidates are, this election is sure to be a real nail-biter in its final moments.