Lossless is formats like Flac and zip. Lossless compression basically stores the same data in more efficient (from a file size perspective) states rather than discarding stuff that isn’t perceived.

The clue is in the name of the term: “lossy” means you lose data. “Lossless” means you don’t lose data. So if a zip file was lossy, you’d never be able to decompress it. Whereas you cannot restore data you’ve lost from an MP3.

Lossless compression is entirely reversible. Nothing is lost and nothing is discarded, perceived or not, like zip.

0: https://arxiv.org/abs/2106.02782