From Longman Dictionary of Contemporary Englishunicycleu‧ni‧cy‧cle /ˈjuːniˌsaɪkəl/ noun [countable] TTBa vehicle that is like a bicycle but has only one wheel
Examples from the Corpus
unicycle• Thirteen years of Tory misrule have left the Right's matrimonial affairs looking about as secure as a drunk on a unicycle.• Will it be the Pope on a unicycle?• They learned how to walk on stilts, ride a unicycle and juggle.• Inside there was a young man riding a unicycle, with balloons and decorations everywhere.• They've got some good cyclists, some good ice hockey players, but not many good unicycle hockey players. 