The Collaborative International Dictionary of English v.0.48:

Residual \Re*sid"u*al\ (r?-z?d"?-al), a. [See Residue.]
   Pertaining to a residue; remaining after a part is taken.
   [1913 Webster]

   Residual air (Physiol.), that portion of air contained in
      the lungs which can not be expelled even by the most
      violent expiratory effort. It amounts to from 75 to 100
      cubic inches. Cf. Supplemental air, under
      Supplemental.

   Residual error. (Mensuration) See Error, 6
   (b) .

   Residual figure (Geom.), the figure which remains after a
      less figure has been taken from a greater one.

   Residual magnetism (Physics), remanent magnetism. See under
      Remanent.

   Residual product, a by product, as cotton waste from a
      cotton mill, coke and coal tar from gas works, etc.

   Residual quantity (Alg.), a binomial quantity the two parts
      of which are connected by the negative sign, as a-b.

   Residual root (Alg.), the root of a residual quantity, as
      [root](a-b).
      [1913 Webster]