The prevalence is “a measure of disease occurrence: the total number of individuals who have an attribute or disease at a particular time (it may be a particular period) divided by the population at risk of having the attribute or disease at that time or midway through the period. When used without qualification, the term usually refers to the situation at a specified point in time (point prevalence). A measure of occurrence or disease frequency, often used to refer to the proportion of individuals in a population who have a disease or condition. It is a proportion, not a rate.”
Porta, M., A dictionary of epidemiology. Fifth Edition ed. 2008, New York: Oxford University Press.
Also see Incidence.