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]

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "diagram":
   alphabet, art, black and white, blueprint, brouillon, cartoon,
   catalog, catalogue raisonne, catch a likeness, chalk, character,
   charactering, characterization, characterize, charcoal,
   charcoal drawing, chart, chiaroscuro, choreography, color,
   conventional representation, copy, crayon, crosshatch,
   dance notation, dash off, daub, delineate, delineation,
   demonstration, depict, depiction, depictment, design, doodle,
   draft, drama, draw, drawing, ebauche, elevation, esquisse,
   exemplification, figuration, figure, graph, ground plan, hatch,
   hieroglyphic, hit off, house plan, ichnography, iconography,
   ideogram, illustration, imagery, imaging, lay off, lay out, letter,
   limn, limning, line drawing, logogram, logograph, map, map out,
   mark off, mark out, musical notation, notate, notation, outline,
   paint, paint a picture, pastel, pattern, pen-and-ink, pencil,
   pencil drawing, pictogram, picture, picturization, picturize, plan,
   plot, plot out, portraiture, portray, portrayal, prefigurement,
   presentment, print, printing, profile, projection, realization,
   register, render, rendering, rendition, represent, representation,
   rough, rough copy, rough draft, rough outline, rub, schema,
   schematize, scheme, score, scratch, script, scumble, set out,
   shade, silhouette, silver-print drawing, sinopia, skeleton, sketch,
   sketch out, stencil, study, syllabary, symbol, symbolize,
   tablature, table, table of contents, take a rubbing, tint, trace,
   trace out, trace over, tracing, vignette, working drawing, write,
   writing