(1) The usual form, or a composite form, that all others of the class resemble more or less closely; a model, denoting especially a disease or a symptom complex giving the stamp or characteristic to a class.constitution, habitus, personality. (2) In chemistry, a substance in which the arrangement of the atoms in a molecule may be taken as representative of other substances in that class. (3) [TA], A specific variation of a structure. SYN: typus [TA], variation (2). [G. typos, a mark, a model]