The Collaborative International Dictionary of English v.0.48:

Diagram \Di"a*gram\, n. [Gr. ?, fr. ? to mark out by lines; dia`
   through + ? to draw, write: cf. F. diagramme. See Graphic.]
   1. (Geom.) A figure or drawing made to illustrate a
      statement, or facilitate a demonstration; a plan.
      [1913 Webster]

   2. Any simple drawing made for mathematical or scientific
      purposes, or to assist a verbal explanation which refers
      to it; a mechanical drawing, as distinguished from an
      artistical one.
      [1913 Webster]

   Indicator diagram. (Steam Engine) See Indicator card,
      under indicator
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diagram \Di"a*gram\, v. t.
   To put into the form of a diagram.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diagram
    n 1: a drawing intended to explain how something works; a
         drawing showing the relation between the parts
    v 1: make a schematic or technical drawing of that shows
         interactions among variables or how something is
         constructed [syn: diagram, plot]