1. a prefix occurring originally in loanwords from Latin (subject; subtract; subvert; subsidy); on this model, freely attached to elements of any origin and used with the meaning "under," "below," "beneath" (subalpine; substratum), "slightly," "imperfectly," "nearly" (subcolumnar; subtropical), "secondary," "subordinate" (subcommittee; subplot).
2. Chem.
a. a prefix indicating a basic compound: subacetate; subcarbonate; subnitrate.
b. a prefix indicating that the element is present in a relatively small proportion, i.e., in a low oxidation state: subchloride; suboxide.
Also, su-, suc-, suf-, sug-, sum-, sup-, sur-, sus-.
[ < L, comb. form repr. sub (prep.); akin to Gk hypó; see HYPO-]

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Universalium. 2010.