The Collaborative International Dictionary of English v.0.48:

Linear \Lin"e*ar\ (l[i^]n"[-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.
      [1913 Webster]

   2. (Bot.) Like a line; narrow; of the same breadth
      throughout, except at the extremities; as, a linear leaf.
      [1913 Webster]

   3. Thinking in a step-by-step analytical and logical fashion;
      contrasted with holistic, i.e. thinking in terms of
      complex interrelated patterns; as, linear thinkers.
      [PJC]

            Linear thinkers concluded that by taking the world
            apart, the actions of people were more predictable
            and controllable.                     --David Morris
                                                  (Conference
                                                  presentation,
                                                  Fairfield
                                                  University,
                                                  October 31,
                                                  1997)

   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.


   Linear measure, the measurement of length.

   Linear numbers (Math.), such numbers as have relation to
      length only: such is a number which represents one side of
      a plane figure. If the plane figure is square, the linear
      figure is called a root.

   Linear problem (Geom.), a problem which may be solved
      geometrically by the use of right lines alone.

   Linear transformation (Alg.), a change of variables where
      each variable is replaced by a function of the first
      degree in the new variable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

linear equation
    n 1: a polynomial equation of the first degree