Definition
An all-time high is the top of the full price history. Above it there are no holders sitting at a loss waiting to get back to even, so there is no old supply overhead. The opposite is an all-time low.
How it is used
Traders watch for a break above an all-time high as a sign of strong demand, and for failures to hold it as a sign of fading demand.
Watch out for
A new high says nothing on its own about how far the move can go. Many highs are followed by sharp pullbacks.