Definitions are the pivotal parts of dictionaries. Every lexicographer always strives to write definitions which are not only accurate but also comprehensible to dictionary users. Nevertheless, as shown in this paper, the definitions used in the current LSP dictionaries have not been able to suit the needs of their users. Therefore, it is necessary to create a typology of definitions for LSP dictionaries which can be referred to by lexicographers when writing definitions. The typology is created by implementing the lexicographical function theory which focuses on satisfying user needs. The typology takes into account both text production and text reception functions, as well as encyclopaedic and foreign language competences of the users. The proposed typology is expected to serve as a model which can be referred to by lexicographers when deciding on how to write definitions for LSP dictionaries in general, and ESP dictionaries in particular.