The process of estimating unmeasured data by assuming that the data will follow the same pattern as already measured data. For example, if your measured data is plotted as a short line on an x, y coordinate system, then you can extrapolate, or guess, that the unmeasured data will simply extend that line.
to estimate an unknown quantity by projecting from the basis of what is already known, but outside the limits of the known data (in contrast, see interpolate).
A method for estimating new data points based on existing measurements, and thereby predicting trends. For example, if the data for 1994 and 1995 are 8 and 9 respectively, one could extrapolate from that data to estimate that in 1996 the figure might be 10. In this case since the extrapolation is over time it is also called Forecasting.