Definition
In instrument flight, CAT (Category) refers to the classification system used to group ILS (Instrument Landing System) approaches by the level of precision and minimum visibility required for landing. The categories — CAT I, CAT II, and CAT III (with sub-types IIIa, IIIb, IIIc) — define progressively lower decision heights and runway visual range minimums, requiring increasingly capable aircraft equipment, crew training, and airport infrastructure as the category number rises.
Plain English
A label that tells you how low the clouds and visibility can be for a particular instrument approach. A higher category number means the approach is approved for worse weather, but it also requires more advanced equipment on the aircraft and at the airport.
Context Anchor
Seen in instrument approach and ILS discussions, especially in references to CAT I, CAT II, and CAT III approach operations.
Derivation
From the Latin categoria, meaning 'a class or division.' In ILS use, it simply means a defined class of approach — each class has its own rules for how low the weather can be before the approach is no longer legal or safe.
Why Pilots Care
Determines whether a flight can continue to a landing or must divert when visibility is low.
Intuition Check
Do not read Category here as just a loose label. In this context, CAT points to a specific approved level of instrument approach operation, not simply a general type or preference.
Example Sentence 1
The crew briefed a CAT I ILS approach because the reported visibility was well above the minimums for that category.
Example Sentence 2
We briefed the CAT I minimums before starting the approach.