The Collaborative International Dictionary of English v.0.48:

Holiness \Ho"li*ness\, n. [AS. h[=a]lignes.]
   1. The state or quality of being holy; perfect moral
      integrity or purity; freedom from sin; sanctity;
      innocence.
      [1913 Webster]

            Who is like thee, glorious in holiness! --Ex. xv.
                                                  11.
      [1913 Webster]

   2. The state of being hallowed, or consecrated to God or to
      his worship; sacredness.
      [1913 Webster]

            Israel was holiness unto the Lord.    --Jer.ii.3.
      [1913 Webster]

   His holiness, a title of the pope; -- formerly given also
      to Greek bishops and Greek emperors.

   Syn: Piety; devotion; godliness; sanctity; sacredness;
        righteousness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

holiness
    n 1: the quality of being holy [syn: holiness, sanctity,
         sanctitude] [ant: unholiness]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "holiness":
   Christianity, Christlikeness, Christliness, angelicalness,
   awesomeness, awfulness, beauty of holiness, blessedness,
   consecration, devotion, devoutness, divineness, divinity, eternity,
   fear of God, glory, godlikeness, godliness, godly-mindedness,
   goodness, hallowedness, heavenliness, heavenly-mindedness,
   holy-mindedness, immutability, ineffability, inenarrability,
   inexpressibility, infinite goodness, infinite justice,
   infinite love, infinite mercy, infinite power, infinite wisdom,
   infinity, inviolability, inviolableness, light, majesty,
   numinousness, odor of sanctity, omnipotence, omnipotency,
   omnipresence, omniscience, omnisciency, otherworldliness, piety,
   piousness, pureness, pureness of heart, purity, righteousness,
   sacredness, sacrosanctity, sacrosanctness, sainthood,
   saintlikeness, saintliness, saintship, sanctitude, sanctity,
   seraphicalness, sovereignty, spiritual-mindedness, spirituality,
   ubiquity, unearthliness, unity, unspeakability, unutterability,
   unworldliness, venerability, venerableness



Easton's 1897 Bible Dictionary:

Holiness
   in the highest sense belongs to God (Isa. 6:3; Rev. 15:4), and
   to Christians as consecrated to God's service, and in so far as
   they are conformed in all things to the will of God (Rom. 6:19,
   22; Eph. 1:4; Titus 1:8; 1 Pet. 1:15). Personal holiness is a
   work of gradual development. It is carried on under many
   hindrances, hence the frequent admonitions to watchfulness,
   prayer, and perseverance (1 Cor. 1:30; 2 Cor. 7:1; Eph. 4:23,
   24). (See SANCTIFICATION.)