What is a 'Tombstone' in NoSQL databases?

Approach: Explain marker for deleted data in append-only systems.

A special marker used in distributed systems to signify that a piece of data has been deleted. It is necessary because data is written to immutable files and can't be 'erased' immediately.

Follow-up: What happens if tombstones are not cleaned up?

Key concepts: lsm trees, soft delete, compaction.