The operation of aircraft is divided into VFR and IFR flight. VFR stands for “Visual Flight Rules” and encompasses a set of regulations designed around the operation of aircraft in VMC, or “Visual Meteorological Conditions.” Broadly speaking, VMC conditions are where the pilot can adequately see where he or she is flying and are defined by a set of regulations surrounding minimum flight visibility and clearance from clouds (or “minimums”). By contrast, IFR stands for “Instrument Flight Rules” and encompasses a set of regulations designed around operation of aircraft in IMC, or “Instrument Meteorological Conditions.” IMC conditions are those below the minimums for VFR flight.
To operate in IMC, a flight needs an IFR-equipped aircraft, and a “instrument rated” pilot. It also needs to be on an “IFR Flight Plan.” The absence of any one of these elements makes it illegal, and extremely dangerous, for a flight to operate in IMC. The term VFR into IMC comes from cases where an aircraft flying VFR inadvertently (or, very occasionally, intentionally) enters IMC without all three elements listed above. The result is often a loss of spatial orientation on the part of the pilot, and a subsequent CFIT crash.