Co-occurrence refers to the frequency of occurrence of two terms from a large and structured set of texts.
In the diagram above, the two circles represent the texts in which each keyword is found. The intersection of these two circles represent those texts where both keywords are found. The intersection is where the co-occurrence occurs.
Co-occurrence in this linguistic sense can be interpreted as an indicator of semantic proximity or an idiomatic expression. In contrast to collocation, which are examples of lexical units, co-occurrence assumes interdependency of the two terms.
Google has patents which use such methodologies to retrieve, index, organize and describe documents:
An information retrieval system uses phrases to index, retrieve, organize and describe documents. Phrases are identified that predict the presence of other phrases in documents. Documents are the indexed according to their included phrases. Related phrases and phrase extensions are also identified. Phrases in a query are identified and used to retrieve and rank documents. Phrases are also used to cluster documents in the search results, create document descriptions, and eliminate duplicate documents from the search results, and from the index.
At Theme Zoom, co-occurrence is one of the foundational concepts behind themes and thematically oriented or silo structured websites.
Using themes, and by extension co-occurrence, you will facilitate the search engine's ability to understand the relationship between the pages on your website and this will allow you to rank easier with fewer inbound links.