Instrument meteorological conditions (IMC) is an aviation flight category that describes weather conditions that require pilots to fly primarily by reference to instruments, and therefore under instrument flight rules (IFR), rather than by outside visual references under visual flight rules (VFR). Typically, this means flying in cloudy or bad weather. Pilots sometimes train to fly in these conditions with the aid of products like Foggles, specialized glasses that restrict outside vision, forcing the student to rely on instrument indications only.
The weather conditions required for flight under VFR are known as visual meteorological conditions (VMC). IMC and VMC are mutually exclusive. In fact, instrument meteorological conditions are defined as less than the minima specified for visual meteorological conditions. The boundary criteria between VMC and IMC are known as the VMC minima. There is also a concept of "marginal VMC", which are certain conditions above VMC minima, which are fairly close to one or more of the VMC minima.
With good visibility, pilots can determine the aircraft attitude by utilising visual cues from outside the aircraft, most significantly the horizon. Without such external visual cues, pilots must use an internal source of the attitude information, which is usually provided by gyroscopically-driven instruments such as the attitude indicator ("artificial horizon"). The availability of a good horizon cue is controlled by meteorological visibility, hence minimum visibility limits feature in the VMC minima.
Because the basic traffic avoidance principle of flying under visual flight rules (VFR) is to "see and avoid", it follows that distance from clouds is an important factor in the VMC minima: as aircraft flying in clouds cannot be seen, a buffer zone from clouds is required (to provide for time to react to an aircraft exiting the clouds).
ICAO recommends the VMC minima internationally; they are defined in national regulations, which rarely significantly vary from ICAO. The main variation is in the units of measurement as different states use different units of measurement in aviation. The minima tend to be stricter in controlled airspace, where there is much traffic therefore greater visibility and cloud clearance is desirable. The degree of separation provided by air traffic control is also a factor. For example, in class A and B airspace where all aircraft are provided with positive separation, the VMC minima feature visibility limits only, whereas in classes C-G airspace where some or all aircraft are not separated from each other by air traffic control, the VMC minima also feature cloud separation criteria.
It is important not to confuse IMC with IFR (instrument flight rules) - 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, for operational reasons or when flying in airspace where flight under VFR is not permitted; indeed by far the majority of commercial flights are operated solely under IFR.
It is possible to be flying VFR in conditions that are legally considered VMC and have to rely on flight instruments for attitude control because there is no distinct external horizon, for example, on a dark night over water (which may create a so-called black hole effect) or a clear night with lights on the water and stars in the sky looking the same.