Plain English Meaning
When the number you need isn't shown in the table but the numbers just above and just below it are, you work out roughly where your number would sit between them.
Full Definition
To estimate a value that falls between two known values in a table or chart by calculating proportionally between them. In aviation performance work, interpolation is used when the actual conditions (temperature, weight, altitude) sit between the values listed in a performance table.
Context Anchor
Seen when using aircraft performance tables, such as takeoff distance or landing distance tables, when the exact condition is not listed.