The term word formation can be understood in several ways. Word can mean 'word-form', a unit defined in writing by preceding and following spaces, and in speech - sometimes - by phonological clues. In this sense, wolf and wolves, ride, rides and rode are five different words. Word can mean 'grammatical word': just as singular wolf and plural wolves or past tense rode and past participle ridden are different grammatical words, so are singular and plural sheep, and past tense walked and past participle walked. And thirdly, word can mean 'lexeme', the unit of vocabulary which subsumes grammatical variants and is their representative in dictionaries - the nouns wolf and sheep, the verbs ride and walk. \Vhen its first element has all three of these senses, word formation is a synonym of morphology, the study of the structure of words.