(countable) An extremely cruel act; a horrid act of injustice.
to carry out / commit / perpetrate an atrocity
The regime is guilty of mass atrocities including forced displacement and the use of chemical weapons.
(uncountable) The quality or state of being atrocious; enormous wickedness; extreme criminality or cruelty.
1553, John Bradford, letter, in Miles Coverdale (ed.), Certain Most Godly, Fruitful, and Comfortable letters, London: John Day, 1564, pp. 481-482,
Thys wil I muse on, & way with my self, [tha]t I may dulye knowe, both in me and in al other things, the atrocitie and bitternesse of synne which dwelleth in me, & so may the more hartely geue ouer my self wholy to [th]e lord Christ my Sauiour,
(countable) An object considered to be extremely unattractive or undesirable.