What is Accusation?

Legal Definition
1) Formally charging a person with a crime either by a prosecuting attorney filing charges against or through a grand jury indictment of that person. 2) Informally stating that a person has committed an illegal or immoral act.
Illustrative caselaw
See, e.g. Rothgery v. Gillespie County, Tex., 554 U.S. 191 (2008) and U.S. v. Patterson, 150 U.S. 65 (1893).
See also
Legal Definition
Crim. law. A charge made to a competent officer against one who has committed a crime or misdemeanor, so that he may be brought to justice and punishment.

2. A neglect to accuse may in some cases be consicleied a misdemeanor, or misprision. (q. v.) 1 Bro. Civ. Law, 247; 2 Id. 389; Inst. lib. 4, tit. 18.

3. It is a rule that no man is bound to accuse himself, or to testify against himself in a criminal case. Accusare nemo se debet nisi coram Deo. Vide Evidence; Interest; Witness.
-- Bouviers Law Dictionary
Legal Definition
A formal charge against a person, to the effect that he is guilty of a punishable offence, laid before a court or magistrate having jurisdiction to inquire into the alleged crime. See Accuse.
-- Black's Law Dictionary
Legal Definition
A charge of the commission of a criminal offence.
-- Ballentine's Law Dictionary