co-occurrence

n. a relation between two or more phenomena (objects or events) such that they tend to occur together. For example, thunder co-occurs with lightning, and in English the letter Q typically co-occurs with the letter U. By itself, co-occurrence does not imply a causal link: For co-occurrence to become covariation, there must be a systematic correlation between both the occurrence of the phenomena and their nonoccurrence. —co-occur vb.