A deadlock in operating system occurs when two or more processes are unable to proceed because each is waiting for a resource held by the other, creating a cycle of dependency. Deadlocks have four necessary conditions: 

1. Mutual Exclusion: Resources cannot be shared.
2. Hold and Wait: Processes hold resources while waiting for others.
3. No Preemption: Resources can't be forcibly taken from a process.
4. Circular Wait: A closed loop of processes exists, each waiting for a resource from the next.

Deadlock prevention, avoidance, detection, and recovery strategies are used to manage and mitigate the risk of deadlocks.