1. [adj]  not drawing attention; "an unnoticeable cigarette burn on the carpet"; "an obscure flaw"
2. [adj]  not noticeable; not drawing attention; "her clothes were simple and unnoticeable"- J.G.Cozzens
3. [adj]  not obtrusive or undesirably noticeable; "a quiet, unobtrusive life of self-denial"

UNNOTICEABLE is a 12 letter word that starts with U.