Learn in the textbookTransaction Fundamentals and Concurrency Control
DefinitionA deadlock (デッドロック) is a state in which several processes each hold a resource while waiting for another to release the resource it holds, so none of them can move forward.
ExplanationSuppose process 1 locks resource X and process 2 locks resource Y, and then process 1 asks for Y while process 2 asks for X: each now waits for the other to unlock, forming a ring. This standoff cannot clear unless one side gives way, so the DBMS resolves it by, for instance, rolling back one process to free its locks.
AnalogyPicture two diners who each hold one chopstick and wait for the other's to free up, so neither can start eating.
Memory hookDead (not moving) plus lock: two parties lock each other out and freeze, just as the word says.