This is how the UK ANO defines it :
Flight within controlled airspace
25 (1) Within Class B airspace:
(a) an aircraft flying within Class B airspace at or above flight level 100 shall remain clear of cloud and in a flight visibility of at least 8 km;
(b) an aircraft flying within Class B airspace below flight level 100 shall remain clear of cloud and in a flight visibility of at least 5 km.
(2) Within Class C, Class D or Class E airspace:
(a) an aircraft flying within Class C, Class D or Class E airspace at or above flight level 100 shall remain at least 1500 metres horizontally and 1000 feet vertically away from cloud and in a flight visibility of at least 8 km;
(b) subject to sub-paragraph (c), an aircraft flying within Class C, Class D or Class E airspace below flight level 100 shall remain at least 1500 metres horizontally and 1000 feet vertically away from cloud and in a flight visibility of at least 5 km;
(c) sub-paragraph (b) shall be deemed to be complied with if:
(i) the aircraft is not a helicopter and is flying at or below 3000 feet above mean sea level at a speed which, according to its airspeed indicator, is 140 knots or less and it remains clear of cloud, in sight of the surface and in a flight visibility of at least 5 km; or
(ii) the aircraft is a helicopter flying at or below 3000 feet above mean sea level and it remains clear of cloud and in sight of the surface.
Flight outside controlled airspace
26 (1) An aircraft flying outside controlled airspace at or above flight level 100 shall remain at least 1500 metres horizontally and 1000 feet vertically away from cloud and in a flight visibility of at least 8 km.
(2) (a) Subject to sub-paragraph (b), an aircraft flying outside controlled airspace below flight level 100 shall remain at least 1500 metres horizontally and 1000 feet vertically away from cloud and in a flight visibility of at least 5 km.
(b) Sub-paragraph (a) shall be deemed to be complied with if:
(i) the aircraft is flying at or below 3000 feet above mean sea level and remains clear of cloud and in sight of the surface and in a flight visibility of at least 5 km;
(ii) the aircraft, other than a helicopter, is flying at or below 3000 feet above mean sea level at a speed which according to its air speed indicator is 140 knots or less and remains clear of cloud and in sight of the surface and in flight visibility of at least 1500 metres; or
(iii) in the case of a helicopter the helicopter is flying at or below 3000 feet above mean sea level flying at a speed, which having regard to the visibility is reasonable, and remains clear of cloud and in sight of the surface.