wan aɪ eɪs aɪ ˈaɪ.əˌdaɪn, -dɪn, -diːn wʌn wan wʌn oʊn ˈsɪŋɡəl ˈjuːnɪtɪ ˈjunɪti the 9th letter of the Roman alphabet the smallest whole number or a numeral representing this number he has the one but will need a two and three to go with it they had lunch at one a nonmetallic element belonging to the halogens; used especially in medicine and photography and in dyes; occurs naturally only in combination in small quantities (as in sea water or rocks)