DEFINITION of expressions and words, as a source of learning, is the main and an indispensable part of academic textbooks. This research aims to apply the concepts of Text Linguistics and Halliday's Functional Theory into the description of Definition that has traditionally been studied from logical and philosophical approaches. The authors try to path the way for the systematic criticism and assessment of academic textbooks through the pathology of a sample of some definitions in such books.After describing Definition, its structure and markers, the article investigates the seven factors of text formation (cohesion, coherence, informativity, textuality, intertextuality, acceptability, and intentionality) and the seven factors of text cohesion (dependence and linking/conjunction, repetition, semantic ties, pronouns, ellipsis, and enumeration) in 70 exemplary definitions that have been randomly gathered from some academic textbooks. This study shows that definition has the structural features of a text, and introduces the most common textual and cohesive factors, the dominant information structure as well as the disordered textual and cohesive factors within the sample definitions.