Similarity

1. An aspect, trait, or feature like or resembling another or another’s
2. The fact that people or thing look or are the same.

some common synonyms for similarity are:

Closeness, comparison, analogy, likeness, association, conformity, homogeneity, identity, parallelism, proportion, relation, similitude, synonymity.

Variation

1. A different form of something; variant.
2. amount, rate, extent; or degree of change.
3. The act, process, or accident of varying in condition, character, or degree.
4. The transformation of a melody or theme with changes or elaborations in harmony, rhythm, and melody.
5. A varied form of a melody or theme, especially one of a series of such forms developing the capacities of the subject.
6. Ballet, a solo dance, especially one forming a section of a pas de deux.

some common synonyms for variation are:

deviation, variety, fluctuation, diversity, alteration, change, alteration, abnormality, contrast, modification, distinction, dissimilarity, novelty, unconformity.