1. (n.) A reasoning that compares the relationships between various objects or events\; A resemblance of relations; an agreement or likeness between things in some circumstances or effects, when the things are otherwise entirely different. Thus, learning enlightens the mind, because it is to the mind what light is to the eye, enabling it to discover things before hidden.

2. (n.) A relation or correspondence in function, between organs or parts which are decidedly different.

3. (n.) Proportion; equality of ratios.

4. (n.) Conformity of words to the genius, structure, or general rules of a language; similarity of origin, inflection, or principle of pronunciation, and the like, as opposed to anomaly.

accordance affinity agent agreement alignment alikeness allegory alliance alternate alternative ambiguity aping approach approximation assimilation backup balancing change changeling closeness coextension collineation community comparability comparative anatomy comparative degree comparative grammar comparative judgment comparative linguistics comparative literature comparative method compare comparing comparison concurrence conformity confrontation confrontment contrast contrastiveness copy copying correlation correspondence counterfeit deputy distinction distinctiveness double dummy equal equidistance equivalent equivocation equivoque ersatz exchange fake fill-in ghost ghostwriter identity imitation likeness likening locum tenens makeshift matching metaphor metonymy mimicking nearness next best thing nondivergence opposing opposition parallelism parity personnel phony pinch hitter proportion proxy relation relief replacement representative resemblance reserves ringer sameness second string secondary semblance sign similarity simile similitude simulation spares stand-in sub substituent substitute substitution succedaneum superseder supplanter surrogate symbol synecdoche tergiversation third string token trope of comparison understudy utility player vicar vice-president vice-regent weighing