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is the process where you "estimate" the points between two (or more) other points.
Let's say you have points 1 and 2 (black), then it's not a problem to calculate where in-between point 3 would be (red).
The process where you try to guess the point to the left of 1 or to the right of 2 is called "extrapolation". It is almost never used in 3D graphics.