ACID is an acronym describing four guarantees that a database provides around how it processes operations: Atomicity, Consistency, Isolation, and Durability. Together they ensure that data remains correct even when many operations run concurrently or when the system fails midway through a write.
Atomicity means a transaction either completes entirely or has no effect at all — there is no partial state. Consistency means every transaction moves the database from one valid state to another, never violating its rules. Isolation means concurrent transactions do not interfere with each other, behaving as if they ran one at a time. Durability means once a transaction is committed, it survives crashes and power loss.
Most purpose-built vector databases offer only partial ACID guarantees. Because they are optimised for high-throughput approximate search rather than transactional correctness, features like multi-document atomic writes are often limited or on the roadmap. This is one reason teams that need both strong transactions and vector search sometimes choose a relational database with a vector extension, accepting slower search in exchange for full ACID compliance.