The term morphology in biology refers to the outward appearance (shape, structure, color, pattern) of an organism or taxon and its component parts. This is in contrast to physiology, which deals primarily with function.
Also in use is the term "gross morphology", which refers to the prominent or principal aspects of an organism or taxon's morphology. A description of an organism's gross morphology would include, for example, its overall shape, overall colour, main markings etc. but not finer details.
Most taxa differ morphologically from other taxa. Typically closely related taxa differ much less than more distantly related ones, but there are exceptions to this. Cryptic species are species which look very similar, or perhaps even outwardly identical, but are reproductively isolated. Conversely, sometimes unrelated taxa acquire similar appearance through convergent evolution or even through mimicry. A further problem with relying on morphological data is that what may appear, morphologically speaking, to be two distinct species, may in fact be shown by DNA analysis to be a single species.
Comparative morphology is comparing the physical aspects of organisms.