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baryon

In particle physics: collective expression for particles that fundamentally consist of three quarks. The most important examples are protons and neutrons.

In cosmology, the word denotes ordinary matter in contrast with more exotic forms of matter that the greatest part of dark matter is thought to consist of. This usage comes about because cosmologists are mainly interested in what percentage of mass in the universe is represented by ordinary matter. The mass of ordinary matter is mostly contained in atomic nuclei, and these nuclei are built of baryons (protons and neutrons), the total baryonic mass is, to good approximation, the same as the total mass of ordinary matter in any region of our universe.