An Etymological Dictionary of Astronomy and Astrophysics

1. Grasp mentally; understand.
   

   2. Include, comprise, or encompass.
      

   Comprehend and understand are very often interchangeable. In fact, both words mean “grasp the meaning of,” but in some cases understand stresses the final result, while comprehend stresses the process of getting there.

See also: From L. comprehendere “to take together, to unite; include; seize,” from → com- “together, with,” + prehendere “to seize,” → prehend.