From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( n.) The lowest point; the time of greatest depression.

(2): ( n.) That point of the heavens, or lower hemisphere, directly opposite the zenith; the inferior pole of the horizon; the point of the celestial sphere directly under the place where we stand.

The Nuttall Encyclopedia [2]

Name given to the part of the heavens directly under our feet, as zenith to that directly over our head.