This might stand for extrapolate, which means to project the data
beyond the confines of your data set (eg you have data for January to
April - project the data to May). A similar term, interpolate, relates
to projecting the data within the data set (eg you have data related
to the integers 1 to 10, and you want to project what the values would
be for 7.5).
Hope this helps.
Pete