The Jargon File (version 4.4.7, 29 Dec 2003):

epsilon squared
 n.

    A quantity even smaller than epsilon, as small in comparison to epsilon
    as epsilon is to something normal; completely negligible. If you buy a
    supercomputer for a million dollars, the cost of the thousand-dollar
    terminal to go with it is epsilon, and the cost of the ten-dollar cable
    to connect them is epsilon squared. Compare lost in the underflow, lost
    in the noise.


The Free On-line Dictionary of Computing (30 December 2018):

epsilon squared

    A quantity even smaller than epsilon, as small in
   comparison to epsilon as epsilon is to something normal;
   completely negligible.  If you buy a supercomputer for a
   million dollars, the cost of the thousand-dollar terminal to
   go with it is epsilon, and the cost of the ten-dollar cable
   to connect them is epsilon squared.

   Compare lost in the underflow, lost in the noise.

   (1997-09-05)