Metamer

Metamer is a term used in several scientific disciplines to describe different notions that share a common theme: the idea that a system can be composed of repeating parts or can appear the same under certain conditions despite differences in structure or composition.

In biology, metamerism refers to the segmentation of an animal’s body into a series of repetitive units called metamers. This organization is characteristic of arthropods, annelids, and some other groups, and can be evident externally as body segments or internally in structures such as the nervous system and musculature. Metamerism is a fundamental aspect of body plan organization in many invertebrates and reflects evolutionary development of repeated modules.

In color science, metamerism describes a perceptual phenomenon in which two color stimuli, with different spectral power distributions, are perceived as the same color under one lighting condition but not under another. Metameric color matches are central to color reproduction in printing, textiles, and lighting, and they depend on the illuminant. Changes in light quality can reveal underlying spectral differences that were not apparent under the reference lighting.

In chemistry, metamers refer to compounds that have the same molecular formula and the same functional groups, but where those groups are arranged differently across the molecule, leading to different physical properties. This conception of metamerism captures differences among constitutionally related species that are not easily described by simple formula alone.

The term thus spans biology, color perception, and chemistry, each using metamer to denote a form of equivalence or modular arrangement within a system.