Ordinals are essentially Bitcoin NFTs you can mint directly on the Bitcoin blockchain.
Each Bitcoin is broken into 100,000,000 units called satoshis (or sats). Each sat is serially numbered, starting at 0. These numbers are "ordinal numbers" in the mathematical sense, giving an order to each sat in the total supply. Satoshi scarcity is cut in half every four years (halving every 210,000 blocks).
Each sat can be inscribed with data, such as JPGs, GIFs, PDFs and more, creating a Bitcoin Ordinal which is then held in a wallet.
Ordinals are interchangeably referred to as Bitcoin Ordinals, Ordinal inscriptions, Bitcoin NFTs, and inscriptions.