The Collaborative International Dictionary of English v.0.48:

Residual \Re*sid"u*al\, n. (Math.)
   (a) The difference of the results obtained by observation,
       and by computation from a formula.
   (b) The difference between the mean of several observations
       and any one of them.
       [1913 Webster]

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]

WordNet (r) 3.0 (2006):

residual
    adj 1: relating to or indicating a remainder; "residual
           quantity" [syn: residual, residuary]
    n 1: something left after other parts have been taken away;
         "there was no remainder"; "he threw away the rest"; "he
         took what he wanted and I got the balance" [syn:
         remainder, balance, residual, residue, residuum,
         rest]
    2: (often plural) a payment that is made to a performer or
       writer or director of a television show or commercial that is
       paid for every repeat showing; "he could retire on his
       residuals"