When a sharp book (Pinnacle, Circa) moves its line in response to a bet, or when a line moves opposite to public ticket count, that's sharp action. Following sharp line moves — rather than public betting percentages — is one of the most durable edges in betting.
Advanced concepts
Sharp money
Bets from professional or winning bettors. Moves lines quickly and reliably.
Example
Lakers open -4, public bets 78% on Lakers, but line moves to Lakers -3. That's sharp money on Suns.