The Collaborative International Dictionary of English v.0.48:

Openness \O"pen*ness\, n.
   The quality or state of being open.
   [1913 Webster]

WordNet (r) 3.0 (2006):

openness
    n 1: without obstructions to passage or view; "the openness of
         the prairies"
    2: characterized by an attitude of ready accessibility
       (especially about one's actions or purposes); without
       concealment; not secretive [syn: openness, nakedness]
       [ant: closeness, secretiveness]
    3: willingness or readiness to receive (especially impressions
       or ideas); "he was testing the government's receptiveness to
       reform"; "this receptiveness is the key feature in oestral
       behavior, enabling natural mating to occur"; "their
       receptivity to the proposal" [syn: receptiveness,
       receptivity, openness]

Moby Thesaurus II by Grady Ward, 1.0:

196 Moby Thesaurus words for "openness":
   access, accessibility, admissibility, admission, amenableness,
   apparentness, approachability, artlessness, assimilation,
   austerity, availability, baldness, bareness, big mouth, bluffness,
   bluntness, broadness, brusqueness, candidness, candor,
   childlikeness, clearness, come-at-ableness, common speech,
   communicativeness, completeness, comprehension, comprehensiveness,
   comprisal, conversableness, coverage, crystal-clearness,
   defenselessness, directness, distinctness, effusion, effusiveness,
   eligibility, embodiment, embracement, encompassment, envisagement,
   evidence, evidentness, exhaustiveness, exposure, extroversion,
   flow of words, flowing tongue, fluency, fluent tongue,
   flux de bouche, flux de paroles, flux of words, forthrightness,
   frankness, freedom, freeness, garrulity, garrulousness, gassiness,
   genuineness, getatableness, gettableness, gift of gab, glibness,
   gregariousness, guilelessness, gush, gushiness, helplessness,
   homespun, hospitality, household words, impressionability,
   inclusion, inclusiveness, incorporation, influenceability,
   ingenuousness, innocence, invitingness, leanness, liability,
   long-windedness, loose tongue, loquaciousness, loquacity,
   malleability, manifestness, matter-of-factness, membership,
   movability, naiveness, naivete, naivety, nakedness, naturalness,
   nonimmunity, obtainability, obtainableness, obviousness, open mind,
   open-mindedness, openheartedness, openness to sight, outgoingness,
   outspokenness, overtness, palpability, participation, patentness,
   penetrability, perspicuity, persuadability, persuadableness,
   persuasibility, perviousness, plain English, plain dealing,
   plain speaking, plain speech, plain style, plain words, plainness,
   plainspokenness, procurability, procurableness, prolixity,
   prosaicness, prosiness, reachableness, reception, receptiveness,
   receptivity, recipience, responsiveness, restrainedness, roundness,
   rustic style, securableness, severity, simpleheartedness,
   simplemindedness, simpleness, simplicity, sincerity,
   single-heartedness, single-mindedness, singleness of heart, slush,
   soberness, sociability, spareness, spate of words, starkness,
   straightforwardness, suggestibility, susceptibility, susceptivity,
   swayableness, talkativeness, tangibility, tolerance, toleration,
   trustfulness, unadorned style, unadornedness, unaffectedness,
   unconstraint, unguardedness, unimaginativeness, unmistakableness,
   unpoeticalness, unquestionability, unrepression, unreserve,
   unreservedness, unrestraint, unrestriction, unreticence,
   unsecretiveness, unsophisticatedness, unsophistication,
   unsuppression, unsuspiciousness, untaciturnity, unwariness,
   verbosity, vernacular, visibility, volubility, vulnerability,
   weakness, whole, windiness