also knife edge
1.
To be on a ~ means to be in a situation in which nobody knows what is going to happen next, or in which one thing is just as likely to happen as another. (mainly BRIT)
The game is poised on a ~. One mistake or one piece of good luck could decide it.
PHRASE: oft v-link PHR
2.
You can use ~ to refer to something that is very exciting or tense because you do not know what is going to happen next. (mainly BRIT)
Tonight’s ~ vote could be uncomfortably close.
ADJ: ADJ n