The Collaborative International Dictionary of English v.0.48:

Marker \Mark"er\, n.
   One who or that which marks. Specifically:
   (a) One who keeps account of a game played, as of billiards.
   (b) A counter used in card playing and other games.
   (c) (Mil.) The soldier who forms the pilot of a wheeling
       column, or marks the direction of an alignment.
   (d) An attachment to a sewing machine for marking a line on
       the fabric by creasing it.
       [1913 Webster]

WordNet (r) 3.0 (2006):

marker
    n 1: some conspicuous object used to distinguish or mark
         something; "the buoys were markers for the channel"
    2: a distinguishing symbol; "the owner's mark was on all the
       sheep" [syn: marker, marking, mark]
    3: a writing implement for making a mark

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "marker":
   accountant, amanuensis, arch, archivist, barrow, beacon, beam,
   bench mark, bookkeeper, bookmark, boundary stone, brass, bust,
   cairn, catstone, cenotaph, clerk, column, cromlech, cross, cup,
   cyclolith, documentalist, dolmen, engraver, fan marker,
   filing clerk, flare path, footstone, grave, gravestone, headstone,
   hoarstone, inscription, landmark, librarian, lighthouse, lightship,
   mark, mausoleum, megalith, memento, memorial, memorial arch,
   memorial column, memorial statue, memorial stone, menhir, milepost,
   milestone, monolith, monument, mound, necrology, notary,
   notary public, obelisk, obituary, pharos, pillar, plaque, platform,
   prize, prothonotary, pylon, pyramid, racon, radar beacon,
   radio beacon, record clerk, recorder, recordist, register,
   registrar, reliquary, remembrance, ribbon, rostral column,
   scorekeeper, scorer, scribe, scrivener, seamark, secretary, shaft,
   shrine, stela, stenographer, stone, stonecutter, stupa, tablet,
   testimonial, timekeeper, tomb, tombstone, tope, tower, trophy,
   watchtower, wind cone, wind indicator, wind sock