Definition: A tempo mark indicating a quick duple meter, often represented by the “cut time” symbol (a C with a vertical line through it). It implies the half note, rather than the quarter note, gets the beat.
The Savvy Take: When you see this, stop counting “1-2-3-4” and start counting “1… 2…”. It changes the entire feel of the piece from a march to a swing. It usually means “play faster and lighter.”