What is Battery?

Legal Definition
At common law, battery is the tort of intentionally (or, in Australia, negligently) and voluntarily bringing about an unconsented harmful or offensive contact with a person or to something closely associated with them (e.g. a hat, a purse). Unlike assault, battery involves an actual contact. The contact can be by one person (the tortfeasor) of another (the victim), or the contact may be by an object brought about by the tortfeasor. For example, the intentional contact by a car is a battery.

Unlike criminal law, which recognizes degrees of various crimes involving physical contact, there is but a single tort of battery. Lightly flicking a person's ear is battery, as is severely beating someone with a tire iron. Neither is there a separate tort for a battery of a sexual nature. However, a jury hearing a battery case is free to assess higher damages for a battery in which the contact was particularly offensive or harmful.

Since it is practically impossible to avoid physical contact with others during everyday activities, everyone is presumed to consent to a certain amount of physical contact with others, such as when one person unavoidably brushes or bumps against another in a crowded lift, passage or stairway. However, physical contact may not be deemed as consented to if the acts that cause harm are prohibited acts.
-- Wikipedia
Legal Definition
1. In criminal law, a physical act that results in harmful or offensive contact with another's person without that person's consent.

2. In tort law, the intentional causation of harmful or offensive contact with another's person without that person's consent.
Illustrative caselaw
See, e.g. Claggett v. State, 670 A.2d 1002 (Md. Ct. Spec. App. 1996).
See also
Legal Definition
It is proposed to consider, 1. What is a battery; 2. When a battery, may be justified.

2. §1. A battery is the unlawful touching the person of another by the aggressor himself, or any other substance put in motion by him. 1 Saund. 29, b. n. 1; Id. 13 & 14, n. 3. It must be either wilfully committed, or proceed from want of due care. Str. 596; Hob. 134; Plowd. 19 3 Wend. 391. Hence an injury, be it never so small, done to the person of another, in an angry, spiteful, rude or insolent manner, as by spitting in his face, or any way touching him in anger, or violently jostling him, are batteries in the eye of the law. 1 Hawk. P. C. 263. See 1 Selw. N. P. 33, 4. And any thing attached to the person partakes of its inviolability if, therefore, A strikes a cane in the hands of B, it is a battery. 1 Dall. 1 14 1 Ch. Pr. 37; 1 Penn. R. 380; 1 Hill's R. 46; 4 Wash. C. C. R. 534 . 1 Baldw. R. 600.

3. - §2. A battery may be justified, 1. on the ground of the parental relation 2. in the exercise of an office; 3. under process of a court of justice or other legal tribunal 4. in aid of an authority in law; and lastly, as a necessary means of defence.

4. First. As a salutary mode of correction. For example: a parent may correct his child, a master his apprentice, a schoolmaster his scholar; 24 Edw. IV.; Easter, 17, p. 6 and a superior officer, one under his command. Keilw. pl. 120, p. 136 Bull. N. P. 19 Bee, 161; 1 Bay, 3; 14 John. R. 119 15 Mass. 365; and vide Cowp. 173; 15 Mass. 347.

5. - 2. As a means to preserve the peace; and therefore if the plaintiff assaults or is fighting with another, the defendant may lay hands upon him, and restrain him until his anger is cooled; but he cannot strike him in order to protect 'the party assailed, as he way in self-defence. 2 Roll. Abr. 359, E, pl. 3.

6. - 3. Watchmen may arrest, and detain in prison for examination, persons walking in the streets by might, whom there is reasonable ground to suspect of felony, although there is no proof of a felony having been committed. 3 Taunt. 14.

7. - 4. Any person has a right to arrest another to prevent a felony.

8. - 5. Any one may arrest another upon suspicion of felony, provided a felony has actually been committed and there is reasonable ground for suspecting the person arrested to be the criminal, and that the party making the arrest, himself entertained the suspicion.

9. - 6. Any private individual may arrest a felon. Hale's P. C. 89.

10. - 7. It is lawful for every man to lay hands on another to preserve public decorum; as to turn him out of church, and to prevent him from disturbing the congregation or a funeral ceremony. 1 Mod. 168; and see 1 Lev. 196; 2 Keb. 124. But a request to desist should be first made, unless the urgent necessity of the case dispenses with it.

11. Secondly. A battery may be justified in the exercise of an office. 1. A constable may freshly arrest one who, in, his view, has committed a breach of the peace, and carry him before a magistrate. But if an offence has been committed out of the constable's sight, he cannot arrest, unless it amounts to a felony; 1 Brownl. 198 or a felony is likely to ensue. Cro. Eliz. 375.

12. - 2. A justice of the peace may generally do all acts which a constable has authority to perform hence he may freshly arrest one who, in his view has broken the peace; or he may order a constable at the moment to take him up. Kielw. 41.

13. Thirdly. A battery may be justified under the process of a court of justice, or of a magistrate having competent jurisdiction. See 16 Mass. 450; 13 Mass. 342.

14. Fourthly. A battery may be justified in aid of an authority in law. Every person is empowered to restrain breaches of the peace, by virtue of the authority vested in him by the law.

15. Lastly. A battery may be justified as a necessary means of defence. 1. Against the plaintiffs assaults in the following instances: In defence of himself, his wife, 3 Salk. 46, his child, and his servant. Ow. 150; sed vide 1 Salk. 407. So, likewise, the wife may justify a battery in defending her husband; Ld. Raym. 62; the child its parent; 3 Salk. 46; and the servant his master. In these situations, the party need not wait until a blow has been given, for then he might come too late, and be disabled from warding off a second stroke, or from protecting the person assailed. Care, however, must be taken, that the battery do not exceed the bounds of necessary defence and protection; for it is only permitted as a means to avert an impending evil, which might otherwise overwhelm the party, and not as a punishment or retaliation for the injurious attempt. Str. 953. The degree of force necessary to repel an assault will naturally depend upon, and be proportioned to, the violence of the assailant; but with this limitation any degree is justifiable. Ld. Raym. 177; 2 Salk. 642.

16. - 2. A battery may likewise be justified in the necessary defence of one's property; if the plaintiff is in the act of entering peaceably upon the defendant's land, or having entered, is discovered, not committing violence, a request to depart is necessary in the first instance; 2 Salk. 641; and if the plaintiff refuses, the defendant may then, and not till then, gently lay hands upon the plaintiff to remove him from the close and for this purpose may use, if necessary, any degree of force short of striking the plaintiff, as by thrusting him off. Skinn. 228. If the plaintiff resists, the defendant may oppose force to force. 8 T. R. 78. But if the plaintiff is in the act of forcibly entering upon the land, or having entered, is discovered subverting the soil, cutting down a tree or the like, 2 Salk. 641, a previous request is unnecessary, and the defendant may immediately lay hands upon the plaintiff. 8 T. R. 78. A man may justify a battery in defence of his personal property, without a previous request, if another forcibly attempt to take away such property. 2 Salk. 641. Vide Rudeness; Wantonness.
-- Bouviers Law Dictionary
Legal Definition
Any unlawful beating or other wrongful physical violence or constraint, inflicted on a human being without his consent 2 Bish. Crim. Law, § 71; Good-rum v. State, 60 Ga. 511; Razor v. Kinsey, 55 III. App. 614; Lamb v. State, 67 Md. 524, 10 Atl. 209, 298; Hunt v. People, 53 111. App. 112; Perkins v. Stein, 94 Ky. 433, 22 S. W. 649, 20 L. R. A. 861. And see Beat. A battery is a willful and unlawful use of force or violence upon the person of another. Pen. Code Cal. § 242; Pen. Code Dak. § 306. The actual offer to use force to the injury of another person is assault; the use of it ls battery; hence the two terms are commonly combined in the term "assault and battery." See Simple battery.
-- Black's Law Dictionary
Legal Definition
Any injury to the person done in an angry, revengeful, rude or ins'olent manner. See 50 Atl. (Del.) 632.
-- Ballentine's Law Dictionary