sameness

1. The quality or condition of being the same.
2. A lack of variety or change; monotony.

noun

1. The quality or condition of being exactly the same as something else: identicalness, identity, oneness, selfsameness. See same/different/compare.
2. The state of being equivalent: equality, equation, equivalence, equivalency, par, parity. See same/different/compare.
3. A tiresome lack of variety: humdrum, monotone, monotonousness, monotony. See change/persist, excite/bore/interest.

Definition: likeness, similarity
Antonyms: contrast, difference, dissimilarity, variableness, variety

The noun has 2 meanings:

Meaning #1: the quality of being alike
  Antonym: difference (meaning #1)

Meaning #2: the quality of wearisome constancy and lack of variety
  Synonym: monotony