Instrument meteorological conditions (IMC) are weather conditions that require aircraft pilots to fly primarily by reference to instruments rather than outside visual references, as they could in Visual meteorological conditions (VMC). National regulations define the boundary between IMC and VMC, based on visibility and either ceiling (on the ground) or distance from cloud. The criteria tend to be stricter in controlled airspace, especially near airports, where there is a lot of traffic and greater visibility and cloud clearance is desirable.
It is important not to confuse IMC with Instrument flight rules (IFR) -- "IMC" describes the actual weather conditions, while "IFR" describes the rules under which the aircraft is flying. Aircraft can (and often do) fly IFR in clear weather.
