ASA SQL User's Guide
Using Transactions and Isolation Levels
Transaction blocking and deadlock
When a transaction attempts to carry out an operation, but is forbidden by a lock held by another transaction, a conflict arises and the progress of the transaction attempting to carry out the operation is impeded or blocked.
Two-phase locking describes deadlock, which occurs when two or more transactions are blocked by each other in such a way that none can proceed.
Sometimes a set of transactions arrive at a state where none of them can proceed. For more information, see Deadlock.