A key-value store in Bitcoin is a simple and efficient type of database used to store and retrieve data by using unique keys to access the stored values.

It stores data as a collection of key-value pairs: a key, which is a unique identifier, and a value, which is the data associated with that key. Each key is unique and maps to a specific value, allowing for the efficient retrieval, quick access, and management of data.

Some examples include:

wallet.dat: This file contains wallet-related details like private keys and addresses, public keys, and key metadata (labels, HD seed and derivation paths) and uses a key-value store internally.

LevelDB: Bitcoin Core uses the LevelDB key-value storage engine as a high-performance storage library, ensuring that data retrieval and management operations are optimized for the needs of the network.