1. in genetics, a particular kind of mutation characterized by the loss of genetic material from a chromosome. Deletion of a gene or part of a gene may result in the development of a disease or abnormality. The deletion may involve the loss of one or several base pairs (point mutation) or a much larger segment of a chromosome (chromosomal mutation).

2. in generative grammar, the process in which a constituent of the deep structure of a sentence is deleted from the surface structure (i.e., the sentence as used). For example, the sentence I am happy, my mother is too is derived from the deep structure I am happy, my mother is happy too, with the second happy deleted. The question of whether deletion can serve as a psychological model of sentence processing has been a subject of much psycholinguistic investigation. See also ellipsis.