ex·ten·sion / ikˈstenshən/ • n. 1. a part that is added to something to enlarge or prolong it; a continuation: the railroad's southern extension. ∎  a room or set of rooms added to an existing building. ∎  the action or process of becoming or making something larger: the extension of the president's powers. ∎  an increase in the length of time given to someone to hold office, complete a project, or fulfill an obligation. ∎  Comput. an optional suffix to a file name, typically consisting of a period followed by several characters, indicating the file's content or function.2. (also extension cord) a length of electric cord that permits the use of an appliance at some distance from a fixed socket. ∎  an extra telephone on the same line as the main one.3. [usu. as adj.] instruction by a university or college for students who do not attend full time: extension courses.4. (extensions) lengths of real or artificial hair woven into a person's own hair.5. the action of moving a limb from a bent to a straight position: seizures with sudden rigid extension of the limbs. ∎  the muscle action controlling this: triceps extension. ∎  Ballet the ability of a dancer to raise one leg above the waist, or an instance of this. ∎  Med. the application of traction to a fractured or dislocated limb or to an injured or diseased spinal column to restore it to its normal position. ∎  the lengthening of a horse's stride within a particular gait.6. Physics & Philos. the property of occupying space; spatial magnitude: nature, for Descartes, was pure extension in space.PHRASES: by extension taking the same line of argument further: the disclosures raised serious questions about his credibility and, by extension, the credibility of the company.DERIVATIVES: ex·ten·sion·al / -shənl/ adj.