Special VFR (SVFR) Conditions

Meteorological conditions that are less than those required for basic VFR flight in controlled airspace and in which some aircraft are permitted flight under visual flight rules. (FAR definition)
Meteorological conditions that are less than those required for basic VFR flight in Class B, C, D, or E surface areas and in which some aircraft are permitted flight under Visual Flight Rules. (PCG definition).

Source: 14 CFR Federal Aviation Regulations §1, Pilot Contoller Glossary (PCG)

See also:

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