(Science: epidemiology) in a short time interval (e.g a week), the number of events (e.g. Births in the UK) taking place during the interval is approximately proportional to the length of the interval. The constant of proportionality is the rate at which these events occur. This argument becomes more and more exact as the length of the time interval becomes shorter and shorter. When the time interval has shrunk to an instant, the rate has become an `instantaneous rate. A bad term; just call it a rate.