Merriam-Webster's Collegiate® Dictionary, 11th Edition
ONCE
I. adverb
Etymology: Middle English ones, from genitive of on one
Date: 12th century
one time and no more , at any one time ; under any circumstances ; ever , at some indefinite time in the past ; formerly , by one degree of relationship ,
II. noun
Date: 13th century
one single time ; one time at least ,
III. adjective
Date: 1691
that once was ; former ,
IV. conjunction
Date: 1761
at the moment when ; as soon as