adj

• (now historical) Pertaining to or composed of nitre; having the properties of nitre, bitter.
  "Whate'er the wintry Frost Nitrous prepar'd; the various-blossom'd Spring Put in white Promise forth; and Summer-Suns Concocted strong, rush boundless now to View […]."
• (chemistry) Of, relating to, or derived from nitrogen, especially in which the valence of the nitrogen is lower than that of a corresponding nitric species.
  "In fact, nitric oxide (not to be confused with nitrous oxide, or laughing gas) is one of our primary signalling molecules."
• (chemistry) Of or relating to nitrous acid or its derivatives.
• (mycology, biology) Having a sharp odor like ammonia or nitric acid.

noun

• (dentistry, automotive, informal) Nitrous oxide.
• (automotive, informal) The system in some racing vehicles which pumps nitrous oxide into the engine to improve performance.
  "When a shot this big is being run, it will also be necessary to pull 6-10 degrees of timing out when the nitrous is being run."