The Collaborative International Dictionary of English v.0.48:

Incessant \In*ces"sant\, a. [L. incessans, -antis; pref. in- not
   + cessare to cease: cf. F. incessant. See Cease.]
   Continuing or following without interruption; unceasing;
   unitermitted; uninterrupted; continual; as, incessant
   clamors; incessant pain, etc.
   [1913 Webster]

         Against the castle gate,
         . . . Which with incessant force and endless hate,
         They batter'd day and night and entrance did await.
                                                  --Spenser.

   Syn: Unceasing; uninterrupted; unintermitted; unremitting;
        ceaseless; continual; constant; perpetual.
        [1913 Webster]

WordNet (r) 3.0 (2006):

incessant
    adj 1: uninterrupted in time and indefinitely long continuing;
           "the ceaseless thunder of surf"; "in constant pain";
           "night and day we live with the incessant noise of the
           city"; "the never-ending search for happiness"; "the
           perpetual struggle to maintain standards in a democracy";
           "man's unceasing warfare with drought and isolation";
           "unremitting demands of hunger" [syn: ceaseless,
           constant, incessant, never-ending, perpetual,
           unceasing, unremitting]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "incessant":
   ageless, articulated, catenated, ceaseless, chattering, coeternal,
   concatenated, connected, constant, continual, continued,
   continuing, continuous, cyclical, dateless, direct, endless,
   eternal, eterne, ever-being, ever-durable, ever-during,
   ever-recurring, everlasting, everliving, featureless, frequent,
   gapless, haunting, immediate, immemorial, indestructible, infinite,
   interminable, joined, jointless, linked, machine gun, monotonous,
   never-ceasing, never-ending, nonstop, nonterminating, nonterminous,
   olamic, oscillating, perdurable, perennial, periodic, permanent,
   perpetual, pulsating, rapid, reappearing, recurrent, recurring,
   regular, repeated, repetitive, returning, revenant,
   round-the-clock, running, seamless, sempiternal, serried, smooth,
   stable, staccato, steady, straight, stuttering, sustained,
   thematic, thick-coming, timeless, twenty-four-hour, ubiquitous,
   unbroken, unceasing, unchanging, undifferentiated, unending,
   uniform, unintermitted, unintermittent, unintermitting,
   uninterrupted, unrelieved, unremitting, unstopped, unvarying,
   vibrating, without end