Adult: The reproductively capable (mature), fully formed, usually longest lived, stage of an animals life cycle.
Definition requires input. Note, in some insects the larva or juvenile stages may be longer lived than a short-lived reproductive stage (e.g. dragonflies). In flowering plants, the adult is probably the dominant and reproductive form. Under this definition, the adult stage of macroalgae may be considered to be both the sporophyte or gametophyte (as both form gametes), even though the gametophyte may be microscopic. In asexual species, reproductive capability might be considered to separate larval from adult stages.