Dictionary.net

• The distance of a heavenly body from the zenith.

1874 - Etymological and pronouncing dictionary of the English language
By Stormonth, James, Phelp, P. H.

• n. The distance of a heavenly body from the zenith, being the complement of the altitude when the body is above the horizon, and the depression increased by 90', when the body is below the horizon.

1871 - The Cabinet Dictionary of the English Language