Plain English Meaning

A red sign on the taxiway that tells you to stop before entering an area where your aircraft could disturb the radio signals guiding another aircraft down to the runway in bad weather.

Full Definition

A red taxiway hold sign with white inscription (such as 'ILS') marking the boundary of the ILS critical area — the protected zone near the runway where vehicles and aircraft must hold during low-visibility instrument approaches to prevent interference with the Instrument Landing System signal.

Context Anchor

You may see this sign on a taxiway near a runway, especially at airports with instrument approaches and during low-visibility operations.

Derivation

‘Critical’ comes from Greek kritikos meaning ‘decisive’ — a place where small disturbances . The ILS signal is sensitive, so the area where it can be disrupted is treated as a decisive boundary.