Legislative text is distinguished by the frequency with which it specifies the meaning of the words it employs. More than 25,000 terms are defined in the United States Code alone. In few other contexts is there a perceived need to so carefully and repeatedly clarify meaning. This Article examines the roles played by definitions in a reader's understanding and application of a legislative text; it demonstrates that the effects of defining are not as straightforward as we might assume. The discussion is framed by the distinction between legislation as a communication vehicle and as an instrument of governance. In some cases, definition serves predominantly a communicative purpose; it clarifies the speaker's performative function, investing groups of individuals or instances with rights or obligations. The Article suggest that a better understanding of the effect of definition on a reader's interaction with a text, coupled with an appreciation of the different roles served by definition, will enable legislators to draft more useful definitions and enable interpreters to better apply those definitions.