Any pilot or controller anywhere in the world knows the term tail number. I own a US registered airplane and the number is on the fuselage, but it’s still referred to as a tail number. I’ve had Mexican and Dominican Republic controllers ask for my “tail number.” And most American planes have letters in their registration as well. “Number” isn’t literal. And “tail” isn’t literal. But it’s universally understood and it’s distinguished from “fleet number” which may be on the nose or fuselage.

Over the radio “registration number” is 6 syllables. “Tail number” is three syllables. And brevity is essential on radio frequencies. However far more common is “call sign” which may or not be the “tail number/registration.” I have flown outside the US frequently, mostly the Caribbean and Latin America, and tail number is extremely common. The ICAO flight plan calls for an Aircraft Identification — which is the technically correct term internationally but I have never heard that in spoken radio communication in any country.

The point is that there is some aviation “slang” that is widely accepted and used. While we can get mired in pedantry, it doesn’t really add to the discussion. I don’t think many people discussing aviation topics are confused about the term “tail number.”