Plain English Meaning
Officially shutting something down for good. After it's decommissioned, pilots can't use it anymore and it disappears from the charts.
Full Definition
The formal process of permanently taking a navigation aid, instrument approach procedure, airway, or piece of aviation infrastructure out of service. Once decommissioned, the facility or procedure is no longer published, no longer maintained, and may not be used for navigation or approach guidance.
Context Anchor
Seen in discussions of instrument approaches when older navigation aids or procedures are being removed and replaced by newer systems or procedures.