Plain English Meaning
How long a piece of avionics equipment stays in use before it is replaced. Because magnetic variation slowly changes over the years, equipment that lives in an aircraft for a long time can drift out of step with current magnetic conditions if its data isn't refreshed.
Full Definition
The full operational service period of a piece of avionics equipment, from installation through eventual retirement or replacement. In the context of magnetic variation, it refers to the span of years that an avionics unit remains in service, during which Earth's magnetic variation may shift enough to affect the accuracy of stored magnetic reference data unless the equipment is updated.
Context Anchor
Seen in discussions about magnetic variation, databases, and how aircraft electronic as the Earth’s slowly changes.