In swimming, what does the acronym "IM" stand for?

In swimming, the acronym "IM" stands for Individual Medley.

An individual medley is a race in which one swimmer completes all four competitive strokes in a fixed order: butterfly, backstroke, breaststroke, and freestyle. Each stroke covers one-quarter of the total distance. In a 200-metre IM, for example, the swimmer completes 50 metres of each stroke; in a 400-metre IM, each section is 100 metres.

IM is different from a medley relay. In an individual medley, one swimmer performs all four strokes. In a relay, four swimmers each perform one stroke, and the order changes to backstroke, breaststroke, butterfly, and freestyle. The abbreviation is commonly written as IM or I.M. in race schedules and results.

The stroke order is a frequent source of mistakes. Butterfly comes first in the individual event, while backstroke starts the medley relay. Freestyle is the final leg, and under the rules it means the swimmer may use any style other than backstroke, breaststroke, or butterfly.