Definitions
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. The complement, in spherical coordinates, of a latitude (the difference between a latitude and 90°).
from the GNU version of the Collaborative International Dictionary of English
 n. The complement of the latitude, or the difference between any latitude and ninety degrees.
from The Century Dictionary and Cyclopedia
 n. The complement of the latitude—that is, the difference between the latitude, expressed in degrees, and 90°.
Etymologies
co + latitude (Wiktionary)
Examples

The angle θ1 is the colatitude (polar) angle of the rotated vector r1 and hence is the angle with the rotated vector r2, which lies along the zaxis.

The spherical polar angles are the colatitude angle θ and the longitudinal (azimuthal) angle φ.
fbharjo commented on the word colatitude
a horizon definer
January 7, 2009