This definition comes from the technical glossary.
The root node of a merkle tree, a descendant of all the hashed pairs in the tree. Block headers must include a valid merkle root descended from all transactions in that block.
Merkle tree (the tree of which the merkle root is the root node)
Merkle block (a partial merkle branch connecting the root to one or more leaves transactions)
Merkle root — Developer Guide
Merkle trees — Bitcoin Wiki
What is the merkle root? — Bitcoin StackExchange
Merkle tree — Wikipedia