What is MVFR?

Of course, as others here will probably mention, it is incorrect to add
adjectives and qualifications to the acronyms VFR and IFR. These, as Peter
states, are two different sets of operating rules, and it must always be
perfectly clear which one applies at any moment in flying. It's one or
the other - no "marginal" "low" or anything else. The acronyms used to
describe conditions (as opposed to flight rules) are VMC and IMC, visual or
instrument Meteorological Conditions.

The only exception is SVFR - for "Special" VFR, which is correct because it
actualy refers to a specific set of operating rules for pilots and controlers.

All of us probably confuse and abuse sometimes, and it's no crime to say
conditions are VFR, IFR or marginal VFR - but in practice there is no room for
ambiguity. We're expected to know the rules and adhere to them.

G Faris