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  1. Prefix denoting straight, normal, in proper order.
  2. In chemistry, italicized prefix denoting that a compound has two substitutions on adjacent carbon atoms in a benzene ring. For terms beginning ortho- or o-, see the specific name.
  3. The most hydrated of a series of oxoacids, orthophosphoric acid, H3PO4.

[G. orthos, correct]

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