The present chapter deals with intensional definitions, which are one of the most widely used types of definition. While it is necessary to discuss the relevant theoretical background, the present chapter is mainly meant to be a concise, yet comprehensive practical tool for language professionals and domain experts alike. They often have the task of writing intensional definitions in various text production and terminology management settings. Consequently, the present chapter serves a twofold purpose: (1) to summarize the state-of-the-art knowledge on intensional definitions from a terminology science and terminology management point of view and (2) to assist language professionals and domain experts in writing and assessing intensional definitions in their daily work. Also, future options to semi-automatically exploit intensional definitions are outlined.