Instrument meteorological conditions (IMC) are meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling, less than the minima specified for visual meteorological conditions (VMC). (ICAO Annex 2)
VMC are detailed in ICAO Annex 2: Rules of the Air, Chapter 4: Visual Flight Rules. Essentially, they are:
When above 3,000ft or 1,000ft above terrain, whichever is higher:
1500m horizontally and 1,000 ft vertically from cloud;
Flight visibility 5km below 10,000ft and 8km above 10,000ft.
When below 3,000ft or 1,000ft above terrain, whichever is higher: