Extrapolation

(1) An Academic Fanzine by Tom Clareson

A long-running academic zine edited by Tom Clareson which made most sercon fanzines look positively faaanish.

(2) A Definition

from Fancyclopedia 2 ca. 1959
(DeCamp) Prediction from present knowledge and trends, or speculation based thereon, as distinguished from mere guessing; but always keeping the imagination consistent with the knowledge that one started from. This, of course, is just what we have in science-fiction. There are few really impressive examples; Verne is very weak (about like a modern predicting a landing on the moon shortly). [You people who are reading this at Tycho Station, kindly remember that we write in mid-1959.] The bomb story that got ASF raided was on a par with this; there'd been free speculation in prozines (and even comic books) about the explosive virtues of U235. Perhaps the Future Histories of Heinlein and Poul Anderson may be considered examples of extrapolation.
from Fancyclopedia 1 ca. 1944
The writer doesn't find this in his Winston dictionary, but it's a frequent word among scientifictionists. It means to take known data or principles and speculate beyond them, but always keeping the imagination consonant with the knowledge that one started from; this of course is just what is done in science-fiction.