The proportion of a population having a disorder during a specified period of time. The proportion affected on a specified day. The proportion affected during a period such as a week, month, or year. The porportion of a population which has ever had the disorder.
Crude rate is number of cases over population at risk.
One must be careful to define numerator and demominator of any rate. Specific rate is the rate for s sub-population. Eg age standardized rate assumes particular age distribution. It is a measure of incidence. Relative risk is a ratio formed from comparison of two levels of risk.