Definition
An ice accumulation rate on an aircraft in which even short encounters become potentially hazardous, and use of anti-icing or de-icing equipment, or a course change to exit the icing conditions, is necessary.
Plain English
Ice is building up on the airplane fast enough that you can't just sit in it. You need to turn on your ice protection systems or change altitude or heading to get out of the icing area soon.
Context Anchor
Seen in aviation weather products such as AIRMETs and pilot reports when icing conditions are being described.
Derivation
Moderate comes from Latin words meaning kept within measure or in the middle range. That helps here because moderate icing is not the worst icing category, but it is also not minor or safe to ignore.
Why Pilots Care
It signals conditions that can rapidly degrade aircraft performance and controllability if the pilot does not act.
Grounding Statement
Picture looking outside and seeing ice continue to build on the wing or windshield after only a short time in cloud.
Intuition Check
Moderate does not mean comfortable or acceptable here. In icing reports, moderate means hazardous enough that the pilot needs to take action to get out of it or manage it with approved equipment.
Example Sentence 1
The AIRMET Zulu warned of moderate icing between 8,000 and 14,000 feet, so the pilot requested a lower altitude to stay below the freezing level.
Example Sentence 2
After encountering moderate icing on approach, the crew diverted to an airport with warmer temperatures.