The Collaborative International Dictionary of English v.0.48:

Monolith \Mon"o*lith\, n. [F. monolithe, L. monolithus
   consisting of a single stone, Gr. ?; mo`nos single + li`qos
   stone.]
   A single stone, especially one of large size, shaped into a
   pillar, statue, or monument.
   [1913 Webster]

WordNet (r) 3.0 (2006):

monolith
    n 1: a single great stone (often in the form of a column or
         obelisk)

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "monolith":
   aa, abyssal rock, arch, barrow, basalt, bedrock, block lava,
   boundary stone, brash, brass, breccia, bust, cairn, cenotaph,
   column, conglomerate, crag, cromlech, cross, cup, cyclolith,
   dolmen, druid stone, festooned pahoehoe, footstone, gneiss,
   granite, grave, gravestone, headstone, hoarstone, igneous rock,
   inscription, lava, limestone, living rock, magma, mantlerock,
   marker, mausoleum, megalith, memento, memorial, memorial arch,
   memorial column, memorial statue, memorial stone, menhir,
   metamorphic rock, monument, mound, necrology, obelisk, obituary,
   pahoehoe, pillar, pillow lava, plaque, porphyry, prize,
   pudding stone, pyramid, regolith, reliquary, remembrance, ribbon,
   rock, ropy lava, rostral column, rubble, rubblestone, sandstone,
   sarsen, schist, scoria, scree, sedimentary rock, shaft,
   shelly pahoehoe, shrine, stela, stone, stupa, tablet, talus,
   testimonial, tomb, tombstone, tope, trophy, tufa, tuff