Identifying examples of formulaic language in text is a non-trivial challenge, but the difficulties can be much alleviated by the use of an appropriate definition. Three types of definition are distinguished. Type (i) lays out an analytic working space. Type (ii) derives from an analysis and represents a theoretical position. Type (iii) locates examples for subsequent analysis. Examples of each type are discussed. Extreme examples of formulaicity (pre-memorized material, political slogans and military bugle calls) are then used to explore the boundaries of the definition of formulaicity as morpheme-equivalence. Addressing the question ‘Do formulaic sequences constrain expression?’ reveals the inherent tension between novelty and formulaicity in balancing processing parsimony and the need to respond appropriately in unique communicative events.