What is Hostility?

Legal Definition
Hostility is seen as form of emotionally charged aggressive behavior. In everyday speech it is more commonly used as a synonym for anger and aggression.

It appears in several psychological theories. For instance it is a facet of neuroticism in the NEO PI, and forms part of personal construct psychology, developed by George Kelly.
-- Wikipedia
Legal Definition
A state of open enmity; open war. Wolff, Dr. de la Rat. ยง1191. Hostility, as it regards individuals, may be permanent or temporary; it is permanent when the individual is a citizen or subject of the government at war, and temporary when he happens to be domiciliated or resident in the country of one of the belligerents; in this latter case the individual may throw off the national character he has thus acquired by residence, when he puts himself in motion, bona fide, to quit the country sine animo revertendi. 3 Rob. Adm. Rep. 12; 3 Wheat. R. 14.

2. There may be a hostile character merely as to commercial purposes, and hostility may attach only to the person as a temporary enemy, or it may attach only to the property of a particular description. This hostile character in a commercial view, or one limited to certain intents and purposes only, will attach in, consequence of having possessions in the territory of the enemy, or by maintaining a commercial establishment there, or by a personal residence, or, by particular modes of traffic, as by sailing under the enemy's flag of passport. 9 Cranch, 191 5 Rob. Adm. Rep.21, 161; 1 Kent Com. 73; Wesk. on Ins. h. t.; Chit. Law of Nat. Index, h. t.
-- Bouviers Law Dictionary
Legal Definition
In the law of nations. A state of open war. "At the breaking out of hostility." 1 Kent, Comm. 60. An act of open war. "When hostilities have commenced." Id. 56. A hostile character. "Hostility may attach only to the person." Id.
-- Black's Law Dictionary