"A situation which arises when two users, each having a lock on one piece of data, attempt to acquire a lock on the other’s piece of data. The SQL Server detects deadlocks, and kills one user’s process." (Karen Paulsell et al, "Sybase SQL Server: Performance and Tuning Guide", 1996)
"A situation that arises when two users, each having a lock on one piece of data, attempt to acquire a lock on the other’s piece. Each user waits for the other to release his or her lock. SQL Server detects deadlocks and kills one user’s process, returning error code 1205." (Patrick Dalton, "Microsoft SQL Server Black Book", 1997)
"A condition that arises when two or more transactions are waiting for one another to release locks." (Peter Gulutzan & Trudy Pelzer, "SQL Performance Tuning", 2002)
"A situation in SQL Server 2000 when two users each have a lock on one piece of data and they attempt to acquire a lock on the other’s piece of data. Each user would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user’s process." (Anthony Sequeira & Brian Alderman, "The SQL Server 2000 Book", 2003)
"A state in which two users or processes cannot continue processing because they each have a resource that the other needs." (Thomas Moore, "EXAM CRAM™ 2: Designing and Implementing Databases with SQL Server 2000 Enterprise Edition", 2005)
"Occurs when multiple connections that hold locks are waiting for locks to be released that are held by the other transactions, that is, the transactions are waiting for each other to release locks." (Sara Morganand & Tobias Thernstrom , "MCITP Self-Paced Training Kit : Designing and Optimizing Data Access by Using Microsoft SQL Server 2005 - Exam 70-442", 2007)
"A specific type of locking problem when concurrent processes are competing for locks. For example, program1 holds a lock on A and is waiting for a lock on B; program2 holds a lock on B and is waiting for a lock on A." (Craig S Mullins, "Database Administration: The Complete Guide to DBA Practices and Procedures" 2nd Ed, 2012)
"A deadlock occurs when two or more user processes each have a lock on a separate page or table and each wants to acquire a lock on the other process’s page or table. The transaction with the least accumulated CPU time is killed and all of its work is rolled back." (Sybase)
"A situation in which two or more users are waiting for data locked by each other." (Oracle)
"A situation where different transactions are unable to proceed, because each holds a lock that the other needs. Because both transactions are waiting for a resource to become available, neither one ever releases the locks it holds." (MySQL)
"Unresolved contention for the use of resources." (IBM)
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