What is a database deadlock and how is it handled?

Deadlock occurs when two transactions wait for each other's locks indefinitely. Example: T1 locks row A, wants B; T2 locks row B, wants A. Detection: wait-for graph with cycles. Resolution: one transaction is chosen as victim and rolled back (based on cost, age, or priority). Prevention: acquire all locks upfront, order lock acquisition consistently, use shorter transactions, lower isolation levels. Most databases detect and resolve automatically.