Interpolation

A value read between measured points is an interpolation. This is usually more defensible because nearby observations support it. Extending a trend beyond the data is extrapolation and is less reliable: the relationship may change outside the measured range.

Values between plotted points may be estimated by interpolation. Predictions beyond the plotted range are extrapolations and are less secure because the relationship might change outside the measured range.

Interpolation means estimating inside the measured range.