This chapter argues that the word definition is problematic as it implies the existence of word boundaries, something that can easily be discarded. The term definition has been widely discussed not only in lexicography, but also in philosophy, logic and linguistics. Much classical literature describes the lexicographer's task as a search for truth, which is an idea found in both philosophy and classical semantics. The application of sources for collecting lexicographical evidence has changed from the lexicographer using his intuition combined with use of manually gathering citations to using corpus evidence from which data such as collocations and examples are extracted and meaning explanations are extrapolated. When working with meaning explanations from a functional perspective, the concept of relevance is highly important: The lexicographic concept of relevance helps lexicographers when deciding which data to include in dictionaries, ensures that data selected are actually connected with dictionary functions, and that data are presented so as to minimise the comprehension-related information costs.