The Collaborative International Dictionary
Metachrosis
Metachrosis \Met`a*chro"sis\, n. [NL., from Gr. ? beyond + ? a
coloring.] (Biol.)
The power of changing color at will by the expansion of
special pigment cells, under nerve influence, as seen in many
reptiles, fishes, etc.
--Cope.
Wiktionary
metachrosis
n. (context zoology English) The ability to change colour by the expansion and contraction of chromatophores