1. the quality of prevailing generally
being widespread

e.g. he was surprised by the prevalence of optimism about the future

2. a superiority in numbers or amount

e.g. a preponderance of evidence against the defendant

Synonym: preponderance

3. (epidemiology) the ratio (for a given time period) of the number of occurrences of a disease or event to the number of units at risk in the population