Definition
In the context of instrument approaches, CAT (Category) refers to the classification of a precision approach by the lowest decision height (DH) and runway visual range (RVR) authorized for that approach. The standard categories are CAT I, CAT II, and CAT III (with subdivisions IIIa, IIIb, IIIc), each permitting progressively lower minimums and requiring correspondingly more capable aircraft equipment, crew qualifications, and ground facilities.
Plain English
CAT tells you how low the clouds and visibility can be while still allowing the approach to be flown. A higher category number means the pilot is allowed to descend closer to the runway in worse weather before needing to see it.
Context Anchor
Seen on instrument approach charts and radar approach figures as CAT A, CAT B, CAT C, or CAT D in the minimums section.
Derivation
Category comes from an older word meaning a class or group. That helps here because CAT groups aircraft by approach speed so pilots use the correct set of approach minimums.
Why Pilots Care
Determines the lowest altitude and visibility a pilot may use to continue an approach, directly affecting whether the runway can be seen in time to land safely.
Intuition Check
CAT does not mean clear-air turbulence here, and it is not the same as ILS CAT I, II, or III. In this radar approach context, CAT means the aircraft approach category used to select the correct minimums.
Example Sentence 1
The crew briefed a CAT I ILS approach with a 200-foot decision height and 1,800 RVR.
Example Sentence 2
Radar vectored the flight to the CAT I ILS with visibility minimums of 1/2 mile.