Dissimilarity, n.
Want of resemblance; unlikeness; dissimilitude; variety; as, the dissimilarity of human faces and forms. Sir W. Jones.
Shades of meaning
Fernald has no entry of its own for dissimilarity; he lists it under difference.
Where he writes about it:
Dissimilarity is between things sharply contrasted; there may be a difference between those almost alike. — from difference