The Collaborative International Dictionary of English v.0.48:

Gravamen \Gra*va"men\, n.; pl. L. Gravamina , E. {Gravamens.
   [L., fr. gravare to load, burden, fr. gravis heavy, weighty.
   See Grave, a.] (Law)
   The grievance complained of; the substantial cause of the
   action; also, in general, the ground or essence of a
   complaint. Bouvier.
   [1913 Webster]