Source : Webster's Revised Unabridged Dictionary (1913)

Linear \Lin"e*ar\ (-[-e]*[~e]r), a. [L. linearis, linearius, fr.
   linea line: cf. F. lin['e]aire. See 3d {Line}.]
   1. Of or pertaining to a line; consisting of lines; in a
      straight direction; lineal.

   2. (Bot.) Like a line; narrow; of the same breadth
      throughout, except at the extremities; as, a linear leaf.

   {Linear differential equation} (Math.), an equation which is
      of the first degree, when the expression which is equated
      to zero is regarded as a function of the dependent
      variable and its differential coefficients.

   {Linear equation} (Math.), an equation of the first degree
      between two variables; -- so called because every such
      equation may be considered as representing a right line.

Source : WordNet®

linear equation
     n : a polynomial equation of the first degree