• (gas) A fluid in the gaseous state having neither independent shape nor volume and being able to expand indefinitely.
  NCI Thesaurus
  U.S. National Cancer Institute, 2021
• (gas) Vapor state of matter; nonelastic fluids in which the molecules are in free movement and their mean positions far apart.
  CRISP Thesaurus
  National Institutes of Health, 2006
• The vapor state of matter; nonelastic fluids in which the molecules are in free movement and their mean positions far apart. Gases tend to expand indefinitely, to diffuse and mix readily with other gases, to have definite relations of volume, temperature, and pressure, and to condense or liquefy at low temperatures or under sufficient pressure. (Grant & Hackh's Chemical Dictionary, 5th ed)
  NLM Medical Subject Headings
  U.S. National Library of Medicine, 2021
• Gas in your digestive system is part of the normal process of digestion. Getting rid of excess gas, either by burping or passing gas (flatus), also is normal. Gas pain may occur if gas is trapped or not moving well through your digestive system.
  Mayo Clinic
  Mayo Foundation for Medical Education and Research