NOUN (2)

1. myths collectively; the body of stories associated with a culture or institution or person;

2. the study of myths;

The Collaborative International Dictionary of English v.0.48:

Mythology \My*thol"o*gy\, n.; pl. Mythologies. [F. mythologie,
   L. mythologia, Gr. myqologi`a; my^qos, fable, myth + lo`gos
   speech, discourse.]
   1. The science which treats of myths; a treatise on myths.
      [1913 Webster]

   2. A body of myths; esp., the collective myths which describe
      the gods of a heathen people; as, the mythology of the
      Greeks.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mythology
    n 1: myths collectively; the body of stories associated with a
         culture or institution or person
    2: the study of myths

The Devil's Dictionary (1881-1906):

MYTHOLOGY, n.  The body of a primitive people's beliefs concerning its
origin, early history, heroes, deities and so forth, as distinguished
from the true accounts which it invents later.