Baryon

A baryon is a subatomic particle, a hadron (a group of elementary particles that experience strong nuclear force) with a non-integer spin. Baryons accumulate aboard starships as a result of warp travel. They can be removed at facilities that perform baryon sweeps, a procedure lethal to organic material.

Captain Jean-Luc Picard was trapped aboard the USS Enterprise-D during a baryon sweep at the Remmler Array in 2369. (TNG: "Starship Mine")

When Quinn transported the USS Voyager to just before the Big Bang, a build-up of baryonic particles was detected outside. (VOY: "Death Wish")

Baryons are heavy subatomic particles, including protons and neutrons. As all compounds in the universe contain these particles, a process which eliminates baryons wouldn't leave much behind. We can assume that the baryon sweep refers to a specific type of heavy subatomic particle created by the subspace field of warp drive, rather than the entire family of them.