Answer
When a quorum read finds a replica with an older version and writes the newer value back to it, that is read repair. A background process that compares replicas using Merkle trees to find and fix differences is called anti-entropy.
Also accepted: Merkle tree, hash trees, hash tree for Merkle trees.
Why
Read repair only fixes keys that get read; anti-entropy covers the rest. A Merkle tree lets two replicas find which ranges differ by exchanging a few hashes instead of all the data.