A verbal contraction was a shortened form of a word such as "can't" for "can not" or "isn't" for "is not". In slang, it was common to use improper contractions such as "ain't" instead of "isn't". (TNG: "A Fistful of Datas")
By 2367, Data still could not use contractions, which (among other things) made William T. Riker realize he was not in the year 2383, but instead in a simulation created by Barash. (TNG: "Future Imperfect") Later, Data would make two contractions ("I'm" and "can't"), but when Riker asks what he said, Data retracts by saying "cannot." However, Riker states that Data said "can't" and therefore used a contraction. (citation needed • edit)
Q occasionally used contractions in regular speech, but as part of his chaotic god title, this was not too often. (citation needed • edit) However, this was never pointed out in the series, unlike Data.