A statement that seems to contradict itself or common sense but, on reflection, reveals a truth.
In Julius Caesar
Cowards die many times before their deaths; / The valiant never taste of death but once.
"Die many times before they die" sounds contradictory, since one dies only once. The meaning emerges on reading: the coward suffers death in advance, with every fear, while the brave man faces it only once.
Act II: The Conspiracy and the Morning of the Ides, Julius Caesar