Definition of supermarginal in US English:

supermarginal

adjective

  • 1Zoology and Anatomy= supramarginal.

  • 2Better or more than marginal in quality or degree; (Economics) involving or achieving a greater than marginal level of profit or value.

  • 3Economics. Of land or ore: of greater profitability than marginal land or ore. Of a farmer, etc.: working such land.

  • 4Politics. Designating a constituency, etc., in which an election is expected to be extremely closely contested, the sitting or outgoing member typically having only a very small majority.

noun

  • 1Zoology. A supermarginal part; especially one of the plates lying above the marginal plates of a starfish arm. rare.

  • 2Politics. A supermarginal constituency or seat.

Origin

Mid 19th century; earliest use found in James Dana (1813–1895), geologist, zoologist, and teacher. From super- + marginal.