The Collaborative International Dictionary of English v.0.48:

Circularity \Cir`cu*lar"i*ty\, n. [LL. circularitas.]
   The quality or state of being circular; a circular form.
   [1913 Webster]

WordNet (r) 3.0 (2006):

circularity
    n 1: the roundness of a 2-dimensional figure [syn:
         circularity, disk shape]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "circularity":
   aduncity, apparent soundness, aquilinity, arching, arcuation,
   casuistry, circling, circuition, circuitousness, circuitry,
   circulation, circumambience, circumambiency, circumambulation,
   circumflexion, circumlocution, circummigration, circumnavigation,
   concameration, concavity, convexity, convolution, crookedness,
   curvation, curvature, curving, curvity, decurvation, decurvature,
   deviance, deviancy, deviation, deviousness, digression,
   disingenuousness, distortion, equivocalness, equivocation,
   evasive reasoning, excursion, excursus, excurvation, excurvature,
   fallaciousness, fallacy, gyre, gyring, hookedness, incurvation,
   incurvature, incurvity, indirection, insincerity, jesuitism,
   jesuitry, meandering, misapplication, mystification, obfuscation,
   obscurantism, orbit, orbiting, oversubtlety, perversion,
   philosophism, plausibility, plausibleness, rationalization,
   recurvation, recurvature, recurvity, rondure, rotundity,
   roundaboutness, rounding, sinuosity, sinuousness, sophism,
   sophistical reasoning, sophistication, sophistry, special pleading,
   speciosity, specious reasoning, speciousness, spiral, spiraling,
   subtlety, tortuosity, tortuousness, turn, turning, vaulting,
   vicious circle, vicious reasoning, wheeling