Variation. Definition. noun, plural: variations. (1) A difference or deviation (e.g. in structure, form, function) from the recognized norm or standard. (2) A modification in structure, form or function in an organism, deviating from other organisms of the same species or group.(3) Mutant; an organism possessing characteristic(s) as a result of mutation.oun, plural: variations. (1) A difference or deviation (e.g. in structure, form, function) from the recognized norm or standard.