What is a deadlock and what are its four necessary conditions?

A deadlock is a situation where two or more processes are blocked forever, each waiting for resources held by the others. Four necessary conditions (Coffman conditions): Mutual Exclusion (resources are non-sharable), Hold and Wait (process holds resources while waiting for others), No Preemption (resources cannot be forcibly taken), Circular Wait (circular chain of processes waiting for each other). All four must hold for deadlock to occur.