Source : Webster's Revised Unabridged Dictionary (1913)

Mesologarithm \Mes`o*log"a*rithm\, n. [Meso- + logarithm : cf.
   F. m['e]sologarithme.] (Math.)
   A logarithm of the cosine or cotangent. [Obs.] --Kepler.
   Hutton.