Short for Instrument Meteorological Conditions - weather which requires aircraft to be operated under Instrument Flight Rules. The list of weather conditions which indicate whether IMC is in effect is known as VFR minimums; if conditions are not within those minimums, then IMC is in effect.
there are two components to determining if IMC exists. The first is visibility, and it is generally expressed in the regulations and therefore on weather reports in statute miles. The second is cloud clearance - if it is possible for an airplane to remain outside the described minimum distance from clouds. Note that VFR minimums - and hence whether IMC is in effect - will vary in the U.S. not only based on the type of airspace involved but sometimes on whether it is day or night.
(IN 5 22/30)