nos

Here no is a vote or a refusal, and the plural is what gets counted when answers are tallied: the nos carried it, a clear majority of nos. Its spelling collides with the abbreviation for "numbers", a different word built from No. with a period added to the count. In practice the two rarely blur, because a numeral following the word signals the abbreviation.

The short way to write "numbers" before a run of figures: item nos 220 and 221, volume nos 3 to 7. It is the abbreviation No. made plural with -s, with the period dropped, which is why it can look exactly like the plural of the "no" vote. Place it directly before the figures it counts, and the context does the rest.

The gas rather than the machine: N₂O, the anesthetic of surgery and dentistry that motorsport also uses for bursts of added power. As a substance it is uncountable, so you top up the nos or sell nos in canisters rather than buying a nos. Nox is another short form for the same gas.

Racing shorthand for the hardware itself, the system that pumps nitrous oxide into an engine for a burst of performance. It is countable: a tuner wires up a nos, and an inspector can flag one. That is the quick test against the gas it delivers, which is uncountable. The system takes a or the; the gas does not take a. Both uses are informal.