The Collaborative International Dictionary of English v.0.48:

Diameter \Di*am"e*ter\, n. [F. diam[`e]tre, L. diametros, fr.
   Gr. ?; dia` through + ? measure. See Meter.]
   1. (Geom.)
      (a) Any right line passing through the center of a figure
          or body, as a circle, conic section, sphere, cube,
          etc., and terminated by the opposite boundaries; a
          straight line which bisects a system of parallel
          chords drawn in a curve.
      (b) A diametral plane.
          [1913 Webster]

   2. The length of a straight line through the center of an
      object from side to side; width; thickness; as, the
      diameter of a tree or rock.
      [1913 Webster]

   Note: In an elongated object the diameter is usually taken at
         right angles to the longer axis.
         [1913 Webster]

   3. (Arch.) The distance through the lower part of the shaft
      of a column, used as a standard measure for all parts of
      the order. See Module.
      [1913 Webster]

   Conjugate diameters. See under Conjugate.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diameter
    n 1: the length of a straight line passing through the center of
         a circle and connecting two points on the circumference
         [syn: diameter, diam]
    2: a straight line connecting the center of a circle with two
       points on its perimeter (or the center of a sphere with two
       points on its surface)

The Free On-line Dictionary of Computing (19 January 2023):

diameter

   The diameter of a graph is the maximum value of the minimum
   distance between any two nodes.