Uniform — NATO Phonetic Alphabet for U
U = Uniform
Union, not Uniform, was ICAO's first choice for U in the 1951 draft. It didn't survive the round of 1950s international testing that exposed real confusion problems, and Uniform took its place when the standard was finalized in 1956.
Uniform also carries an unusual distinction: ICAO and ITU documentation recognize an alternate pronunciation, "OO-nee-form," alongside the standard English "YOU-nee-form" — one of only a couple of words in the alphabet given two officially acceptable pronunciations rather than one fixed standard.
Its maritime signal flag, hoisted alone, carries a warning rather than a status update: "you are running into danger."
There's a small irony in the choice: "uniform" is also the ordinary English word for standardized military dress, so the same militaries that rely on this alphabet daily use the word constantly in a completely unrelated sense, referring to what a service member wears rather than the letter U. Neither meaning of the word had any bearing on why ICAO's testers picked it for U in the first place.