Consensus mechanisms are covered in full in the entry Consensus, which explains how distributed networks reach agreement and compares the main approaches. This page gives a short definition and explains the two names.
A consensus mechanism is the set of rules a blockchain uses to get thousands of independent computers, which may not trust one another, to agree on a single version of the ledger. It decides who may add the next block, how others check it and what happens when there are competing versions. The best-known mechanisms are proof of work, used by Bitcoin, in which miners spend computing power, and proof of stake, used by Ethereum since 2022, in which validators lock up coins that can be destroyed if they cheat. Other families include delegated proof of stake and voting-based Byzantine fault tolerance protocols, each trading off speed, security and decentralization differently.
"Consensus" is the goal, and "consensus mechanism" or "consensus algorithm" names the method used to reach it. The terms are often used interchangeably.