"On The Definition of Word" develops a consistent and coherent approach to central questions about morphology and its relation to syntax. In sorting out the various senses in which the word "word" is used, it asserts that three concepts which have often been identified with each other are in fact distinct and not coextensive: listemes (linguistic objects permanently stored by the speaker); morphological objects (objects whose shape can be characterized in morphological terms of affixation and compounding); and syntactic atoms (objects that are unanalyzable units with respect to syntax). The first chapter defends the idea that listemes are distinct from the other two notions, and that all one can and should say about them is that they exist. A theory of morphological objects is developed in chapter two. Chapter three defends the claim that the morphological objects are a proper subset of the syntactic atoms, presenting the authors' reconstruction of the important and much-debated Lexical Integrity Hypothesis. A final chapter shows that there are syntactic atoms which are not morphological objects. Anne Marie Di Sciullo is in the Department of Linguistics at the University of Quebec. Edwin Williams is in the Department of Linguistics at the University of Massachusetts. "On The Definition of Word" is Linguistic Inquiry Monograph 14.