The Collaborative International Dictionary of English v.0.48:

Minus \Mi"nus\ (m[imac]"n[u^]s), a. [L. See Minor, and cf.
   Mis- pref. from the French.] (Math.)
   Less; requiring to be subtracted; negative; as, a minus
   quantity.
   [1913 Webster]

   Minus sign (Math.), the sign [-] denoting minus, or less,
      prefixed to negative quantities, or quantities to be
      subtracted. See Negative sign, under Negative.
      [1913 Webster]

WordNet (r) 3.0 (2006):

minus sign
    n 1: a sign indicating the operation of subtraction