Plain English Meaning
Worked out by splitting the difference between two numbers that are shown, to get a number that isn't shown.
Full Definition
Estimated by reading between two known values on a chart, table, or graph to find a value that is not explicitly listed.
Context Anchor
Seen in compass-turn discussions when estimating how much compass error applies to headings between the main reference headings.
Derivation
From Latin 'interpolare', meaning 'to refurbish' or 'insert between'. The 'inter-' part means 'between', which is exactly what you do — you find a value that sits between two given ones.
Why Pilots Care
Performance and compass tables rarely show the exact temperature, weight, or heading you're working with. Interpolating gives a more figure than just rounding to the nearest listed value, which matters for , , and turn headings.