Grounds for legal action

• Formal complaint's mostly serious, so be it
• Basis of a grievance

Wikipedia Word definitions in Wikipedia
Gravamen (from Lat. gravare , to weigh down; gravis , heavy), (plural gra·va·mens or gra·vam·i·na), a complaint or grievance, the ground of a legal action , and particularly the more serious part of a charge against an accused person. In legal terms, the ...

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"grievance," 1640s, from Late Latin gravamen "trouble, physical inconvenience" (in Medieval Latin, "a grievance"), from gravare "to burden, aggravate," from gravis "heavy" (see grave (adj.)).

Wiktionary Word definitions in Wiktionary
n. The grievance complained of; the substantial cause of the action; also, in general, the ground or essence of a complaint.

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
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 ...