In mathematical terms, for any concept c in the taxonomy, let p(c) be the probability of encountering an instance of concept c. Following the standard definition from information theory, the information content of c, IC(c), is then − log p(c).
we can define the semantic similarity of a pair of concepts c 1 and c 2 , as Notice that p is monotonic as one moves up the taxonomy: if c 1 IS-A c 2 then p(c 1 ) ≤ p(c 2 ).