The Collaborative International Dictionary of English v.0.48:

Seamless \Seam"less\, a.
   Without a seam.
   [1913 Webster]

         Christ's seamless coat, all of a piece.  --Jer. Taylor.
   [1913 Webster]

WordNet (r) 3.0 (2006):

seamless
    adj 1: not having or joined by a seam or seams; "seamless
           stockings" [ant: seamed]
    2: smooth, especially of skin; "his cheeks were unlined"; "his
       unseamed face" [syn: seamless, unlined, unseamed]
    3: perfectly consistent and coherent; "the novel's seamless
       plot"

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "seamless":
   articulated, catenated, ceaseless, concatenated, connected,
   constant, continual, continued, continuing, continuous, cyclical,
   direct, endless, featureless, gapless, immediate, incessant,
   interminable, joined, jointless, linked, monotonous, never-ending,
   nonstop, perennial, periodic, recurrent, repetitive,
   round-the-clock, running, serried, smooth, stable, steady,
   straight, twenty-four-hour, unbroken, unceasing, undifferentiated,
   unending, uniform, unintermitted, unintermittent, unintermitting,
   uninterrupted, unrelieved, unremitting, unstopped