Plain English Meaning
Ice forming on the small rods, blades, and wires sticking out of the aircraft that send and receive radio signals. When ice covers them, communication and navigation equipment can become unreliable or stop working properly.
Full Definition
The accumulation of ice on an aircraft's external antennas during flight in icing conditions. Because antennas protrude into the airflow and are usually unheated, they collect ice readily. Ice buildup distorts the antenna's shape and electrical characteristics, which can weaken or block radio signals, degrade navigation reception, and cause vibration or even structural failure of the antenna itself.
Context Anchor
Seen in when the aircraft is in below- or and radio or begin to weaken or become unreliable.