Similarity matrix is the opposite concept to the distance matrix. The elements of a similarity matrix measure pairwise similarities of objects – the greater similarity of two objects, the greater the value of the measure.
For example, the correlation matrix often may be considered as as a similarity matrix of variables – because it is natural to consider pairs of variables with higher values of the correlation coefficient as more similar each to other than pairs with lower values of the correlation coefficient.
See distance matrix for more details.