A blockchain explorer is covered in full in the entry Block Explorer, which explains what explorers show, how they work and how to use them safely. This page gives a short definition and explains the two names.
A blockchain explorer is a website or application that reads data from a blockchain and presents it in a searchable form. Users can look up a transaction by its ID, check the balance and history of a wallet address, view the contents of each block and its block height, and follow tokens and smart contracts. Explorers are read-only: they display public data but cannot move funds. Exchanges and wallets link to them so customers can confirm that a deposit or withdrawal has been recorded, and investigators build more detailed blockchain analytics tools on the same data.
"Block explorer" and "blockchain explorer" mean the same thing. The shorter form is more common among developers, while "blockchain explorer" is often used by newcomers and in search queries. Network-specific names such as "Bitcoin explorer" or "Ethereum explorer" are also used, as was Binance Chain Explorer for the now retired Binance Chain.