To distinguish clearly between words and terms, the first step is to situate general language and language for special purposes. The latter is defined with regard to its users (specialists), its functions (communicating knowledge), and the vocabulary it uses to do so (terms). This enables a clear line to be drawn between lexicology and terminology, positing the existence of lexical units, terminological units, lexical and terminological units, and conceptual units. The relations among these units are examined, and a series of criteria are proposed to distinguish adequately, in regard to their functions of reference and denomination, their relations with meaning and the significance of their form. To do this, terminological units should be considered within a system of concepts and lexical units in a system of language.