M-of-N describes a multisig setup in which N public keys are given joint custody of bitcoin, and M signatures from any of those N keys are required in order to spend the bitcoin.

Transaction malleability is the ability of a transaction to have multiple valid IDs (txids). Malleability occurs when a part of a transaction can change after the transaction has been signed.

The mempool (memory pool) is a smaller database of unconfirmed (pending) transactions which every node keeps. All transactions in the mempool are pending, and should not be trusted until they are included in a block.

A Merkelized Alternative Script Tree (MAST) is a proposal to encapsulate an arbitrary number of different scripts in a Bitcoin address.

The Merkle root is the final node in a Merkle tree. It is a hash which includes the hashes of all transactions in a block.

A Merkle tree is a data structure which stores all transactions in a given block. The Merkle tree's root is placed in the block header as a summary of all transactions.

A Miner-Activated Soft Fork (MASF) is a soft fork which is activated by miners who begin mining blocks under the updated ruleset.

Mining is the process of building the blockchain by adding new blocks one at a time. Most of mining involves finding a valid Proof-of-Work, which is an energy-intensive guessing game.

A mining pool is a cooperative setup which allows smaller miners to pool their hashrate and thereby smooth their revenues. When a member of a pool finds a valid block, they split the rewards with the rest of the pool based on each member's hashrate contribution.

A mixing service accepts bitcoin deposits and sends different pieces of bitcoin back to the user in the same amount. When done properly, this effectively obscures ownership of the bitcoin and allows users to privatize their bitcoin.

A mnemonic phrase is a list of 12-24 words used to back up a wallet. These words can be decoded to yield the seed of a wallet, which generates public and private keys for the wallet.

A multipath payment is a type of Lightning payment which is executed as an atomic set of smaller payments. These smaller payments are more reliable and easier to execute.

Bitcoin can be sent to an address such that spending that bitcoin requires multiple signatures from different private keys. This setup allows joint custody of funds, and can be as simple or as complex as needed.

MuSig is a protocol for creating Taproot multisig public keys and signatures. MuSig makes use of Schnorr signature and public key aggregation