Definition of methine in US English:

methine

noun

Chemistry
  • A trivalent group or radical consisting of a carbon and a hydrogen atom, specifically the group =CH—; also called methene or methenyl. Also (especially in Dye Chemistry): designating a dye, pigment, etc., containing one or more such groups.

Origin

Late 19th century; earliest use found in Henry Roscoe (1833–1915), chemist and university administrator. From meth- + -ine.