(1) A term often used in linguistics (and traditionally used in historical linguistics) as part of a classification of the kinds of element operating within the structure of a word. A root is the base form of a word which cannot be further analyzed without total loss of identity. Putting this another way, it is that part of the word left when all the affixes are removed. From another point of view, roots are sometimes classified as ‘simple’ or ‘complex’/ ‘compound’, though for the latter the term stem is commonly used. From a semantic point of view, the root generally carries the main component of meaning in a word. From a historical viewpoint, the root is the earliest form of a word, though this information is not relevant to a synchronic analysis.
(2) The furthest-back part of the tongue, opposite the pharyngeal wall, not normally involved in the production of speech sounds; also called the radix. It is, however, involved in advanced tongue root articulation - a movement which expands the front-back diameter of the pharynx, used phonologically in some languages as a factor in contrasts of vowel harmony.
(3) In generative grammar, the term is sometimes used to refer to the topmost node in a tree diagram. In non-linear phonology, the root node is the one which dominates all other features in the hierarchy. In transformational grammar it also refers to a type of transformation which applies only to full sentence structure and not to embedded sentences.