The side, or specific area of a pillow, that is neither too warm nor too cold, thus enabling an optimum sleeping or napping process.
Once the individual locates the coordinates of this region of his/her pillow, which usually lies exactly between the extremes of his/her preferred temperature scale, an equilibrium between the extreme temperatures is reached, and he/she will be able to reap the benefits of maximized comfort when at rest. Et voilà!
John couldn't sleep due to the immensely uncomfortable warmth of his pillow. Following careful examination of his pillow's make and year, I informed him of the benefits of locating pillowquibrium.
Thanks to my advice, he now sleeps soundly, and no longer draws comparisons between his area of slumber and the deepest echelons of Hell itself.