Prevalence is the proportion of a population found to have a condition (typically a disease or a risk factor such as smoking). It is calculated by comparing the number of people found to have the condition with the total number of people studied, and is usually expressed as a fraction (for example, 1/3), as a percentage (%) or as the number of cases per 10,000 or 100,000 people.

Prevalence can be measured at a particular point in time (point prevalence), or over a specified period such as a year (period prevalence).