Blockchain bridges are covered in full in the entry Crypto Bridge, which explains how bridges work, the main designs and the security record behind some of crypto's largest hacks. This page gives a short definition and explains the different names.
A blockchain bridge is a system that lets users move tokens, or send messages, from one blockchain to another. The most common design locks tokens in a contract on the original chain and issues an equivalent wrapped token on the destination chain, which is burned when the user moves back. Other designs use pools of liquidity on each side or verify one chain's block headers on the other. Bridges are a key part of blockchain interoperability but concentrate large amounts of value in a few contracts, making them frequent targets for attackers.
"Bridges", "blockchain bridges", "crypto bridges" and "cross-chain bridges" are all used for the same thing. Some providers prefer broader terms such as cross-chain protocol or messaging layer when their system moves data as well as tokens.