IMC
Instrument Meteorological Conditions
Weather conditions below VFR minimums where flight requires instrument reference.
Instrument Meteorological Conditions are weather conditions below the visibility and cloud-clearance minimums required for visual flight rules (VFR). In IMC, a pilot cannot maintain visual reference to the ground or horizon and must navigate and control the aircraft by reference to instruments.
IMC is what an instrument rating exists for. Flight in IMC under instrument flight rules (IFR) requires an instrument rating, current instrument proficiency, an IFR-equipped aircraft, and an ATC clearance.
The boundary between VFR and IMC is set by 14 CFR 91.155 — three statute miles visibility and specified cloud clearance for most controlled airspace. Below those minimums you're in IMC whether you intended to be there or not.
When it matters
Currency for flight in IMC is FAR 61.57(c): six approaches plus holding within the preceding six months, or an IPC with a CFII to restore currency.