Definition
The Merkle root is a cryptographic summary of every transaction in a block. It is computed by recursively hashing pairs of transaction IDs (TXIDs) in a binary tree structure until a single 32-byte hash remains. This root hash is included in the block header.
Merkle trees allow efficient verification: a node can prove a specific transaction is included in a block by providing only a handful of hashes (the Merkle proof) rather than the entire block. This is critical for lightweight SPV (Simplified Payment Verification) clients.
In Simple Terms
A hash summarizing all transactions in a block using a tree structure for efficient verification.
A single hash that represents all transactions in a block, computed by hashing pairs of transaction hashes up a tree structure until one root hash remains.