The Collaborative International Dictionary of English v.0.48:

benchmark \benchmark\, bench mark \bench mark\ (Surveying)
   1. Any permanent mark to which other levels may be referred.
      such as:
      (a) A horizontal mark at the water's edge with reference
          to which the height of tides and floods may be
          measured.
      (b) a surveyer's mark on a permanent object of
          predetermined position and elevation used as a
          reference point.
          [Webster 1913 Suppl. + WordNet 1.5]

   2. something serving as a standard by which related items may
      be judged; as, his painting sets the benchmark of quality.
      [PJC + WordNet 1.5]

   3. a test or series of tests designed to compare the
      qualities or performance of different devices of the same
      type. Certain sets of computer programs are much used as
      benchmarks for comparing the performance of different
      computers, especially by comparing the time it takes to
      complete a test.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

Bench \Bench\, n.; pl. Benches. [OE. bench, benk, AS. benc;
   akin to Sw. b[aum]nk, Dan b[ae]nk, Icel. bekkr, OS., D., & G.
   bank. Cf. Bank, Beach.]
   1. A long seat, differing from a stool in its greater length.
      [1913 Webster]

            Mossy benches supplied the place of chairs. --Sir W.
                                                  Scott.
      [1913 Webster]

   2. A long table at which mechanics and other work; as, a
      carpenter's bench.
      [1913 Webster]

   3. The seat where judges sit in court.
      [1913 Webster]

            To pluck down justice from your awful bench. --Shak.
      [1913 Webster]

   4. The persons who sit as judges; the court; as, the opinion
      of the full bench. See King's Bench.
      [1913 Webster]

   5. A collection or group of dogs exhibited to the public; --
      so named because the animals are usually placed on benches
      or raised platforms.
      [1913 Webster]

   6. A conformation like a bench; a long stretch of flat
      ground, or a kind of natural terrace, near a lake or
      river.
      [1913 Webster]

   Bench mark (Leveling), one of a number of marks along a
      line of survey, affixed to permanent objects, to show
      where leveling staffs were placed. See bench mark in the
      vocabulary.

   Bench of bishops, the whole body of English prelates
      assembled in council.

   Bench plane, any plane used by carpenters and joiners for
      working a flat surface, as jack planes, long planes.

   Bench show, an exhibition of dogs.

   Bench table (Arch.), a projecting course at the base of a
      building, or round a pillar, sufficient to form a seat.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bench mark
    n 1: a surveyor's mark on a permanent object of predetermined
         position and elevation used as a reference point [syn:
         benchmark, bench mark]

Moby Thesaurus II by Grady Ward, 1.0:

78 Moby Thesaurus words for "bench mark":
   abode, area, bearings, bookmark, cairn, cardinal point, catstone,
   chief thing, climax, core, cornerstone, crisis, critical point,
   crux, district, emplacement, essence, essential, essential matter,
   fundamental, gist, gravamen, great point, heart, high point, hole,
   important thing, issue, kernel, keystone, landmark,
   latitude and longitude, lieu, lighthouse, lightship, locale,
   locality, location, locus, main point, main thing, mark, marker,
   material point, meat, menhir, milepost, milestone, monument, nub,
   pharos, pinpoint, pith, pivot, place, placement, platform, point,
   position, real issue, region, salient point, seamark, sine qua non,
   site, situation, situs, spot, stead, substance, substantive point,
   the bottom line, the point, tower, turning point, watchtower,
   whereabout, whereabouts