In one embodiment, generating an ontology includes accessing an inverted index that comprises inverted index lists for words of a language. An inverted index list corresponding to a word indicates pages that include the word. A word pair comprises a first word and a second word. A first inverted index list and a second inverted index list are searched, where the first inverted index list corresponds to the first word and the second inverted index list corresponds to the second word. An affinity between the first word and the second word is calculated according to the first inverted index list and the second inverted index list. The affinity describes a quantitative relationship between the first word and the second word. The affinity is recorded in an affinity matrix, and the affinity matrix is reported.