The Collaborative International Dictionary of English v.0.48:

Keystone \Key"stone`\, n. (Arch.)
   The central or topmost stone of an arch. This in some styles
   is made different in size from the other voussoirs, or
   projects, or is decorated with carving. See Illust. of
   Arch.
   [1913 Webster]

WordNet (r) 3.0 (2006):

keystone
    n 1: a central cohesive source of support and stability; "faith
         is his anchor"; "the keystone of campaign reform was the
         ban on soft money"; "he is the linchpin of this firm" [syn:
         anchor, mainstay, keystone, backbone, linchpin,
         lynchpin]
    2: the central building block at the top of an arch or vault
       [syn: keystone, key, headstone]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "keystone":
   apse, arcade, arcature, arch, arched roof, archway, basis,
   bench mark, camber, cardinal point, ceilinged roof, chief thing,
   climax, concameration, concha, core, cornerstone, cove, crisis,
   critical point, crux, cupola, dome, essence, essential,
   essential matter, first stone, footstone, foundation,
   foundation stone, fundamental, geodesic dome, gist, gravamen,
   great point, headstone, heart, high point, igloo, important thing,
   issue, kernel, landmark, main point, main thing, material point,
   meat, milestone, necessity, nub, ogive, pith, pivot, principle,
   quoin, real issue, salient point, sine qua non, skewback, span,
   substance, substantive point, the bottom line, the point,
   turning point, vault, vaulting, voussoir



U.S. Gazetteer Places (2000):

Keystone, CO -- U.S. Census Designated Place in Colorado
   Population (2000):    825
   Housing Units (2000): 2606
   Land area (2000):     40.636910 sq. miles (105.249108 sq. km)
   Water area (2000):    0.006514 sq. miles (0.016872 sq. km)
   Total area (2000):    40.643424 sq. miles (105.265980 sq. km)
   FIPS code:            40550
   Located within:       Colorado (CO), FIPS 08
   Location:             39.604233 N, 105.948111 W
   ZIP Codes (1990):     80435
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Keystone, CO
    Keystone


U.S. Gazetteer Places (2000):

Keystone, FL -- U.S. Census Designated Place in Florida
   Population (2000):    14627
   Housing Units (2000): 5709
   Land area (2000):     36.078818 sq. miles (93.443706 sq. km)
   Water area (2000):    3.164101 sq. miles (8.194984 sq. km)
   Total area (2000):    39.242919 sq. miles (101.638690 sq. km)
   FIPS code:            36462
   Located within:       Florida (FL), FIPS 12
   Location:             28.127550 N, 82.589117 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Keystone, FL
    Keystone


U.S. Gazetteer Places (2000):

Keystone, IA -- U.S. city in Iowa
   Population (2000):    687
   Housing Units (2000): 281
   Land area (2000):     0.370289 sq. miles (0.959043 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.370289 sq. miles (0.959043 sq. km)
   FIPS code:            41115
   Located within:       Iowa (IA), FIPS 19
   Location:             42.000915 N, 92.198544 W
   ZIP Codes (1990):     52249
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Keystone, IA
    Keystone


U.S. Gazetteer Places (2000):

Keystone, SD -- U.S. town in South Dakota
   Population (2000):    311
   Housing Units (2000): 209
   Land area (2000):     2.864807 sq. miles (7.419816 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    2.864807 sq. miles (7.419816 sq. km)
   FIPS code:            33820
   Located within:       South Dakota (SD), FIPS 46
   Location:             43.893744 N, 103.426080 W
   ZIP Codes (1990):     57751
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Keystone, SD
    Keystone


U.S. Gazetteer Places (2000):

Keystone, WV -- U.S. city in West Virginia
   Population (2000):    453
   Housing Units (2000): 236
   Land area (2000):     0.324252 sq. miles (0.839808 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.324252 sq. miles (0.839808 sq. km)
   FIPS code:            43516
   Located within:       West Virginia (WV), FIPS 54
   Location:             37.415119 N, 81.446082 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Keystone, WV
    Keystone