What is Alien Enemy?

Legal Definition
In international law. An alien who is the subject or citizen of some hostile state or power. See Dyer, 2b; Co.. Litt. 129b. A person who, by reason of owing a permanent or temporary allegiance to a hostile power, becomes, in time of war, impressed with the character of an enemy, and, as such, is disabled from suing in the courts of the adverse belligerent. See 1 Kent, Co.mm 74; 2 Id. 63; Bell v. Chapman. 10 Johns. (N. Y.) 183: Dorsey v. Brigham, 177 111 250, 52 N. E. 307, 42 L. R. A. 809, 69 Am. St. Rep 228.
-- Black's Law Dictionary
Legal Definition
An alien, that is, a citizen or subject of a foreign state or power, residing within a given country, is called an "alien ami" if the country where he lives ls at peace with the country of which he is a citizen or subject; but if a state of war exists between the two countries, he is called an "alien enemy," and in that character is denied access to the courts or aid from any of the departments of government.
-- Black's Law Dictionary
Legal Definition
An alien whose nation is at war with ours.
-- Ballentine's Law Dictionary