What is Assault?

Legal Definition
In criminal and civil law, assault is an attempt to initiate harmful or offensive contact with a person, or a threat to do so. It is distinct from battery, which refers to the actual achievement of such contact.

An assault is carried out by a threat of bodily harm coupled with an apparent, present ability to cause the harm. It is both a crime and a tort and, therefore, may result in either criminal and/or civil liability. Generally, the common law definition is the same in criminal and tort law. There is, however, an additional criminal law category of assault consisting of an attempted but unsuccessful battery. The term is often confused with battery, which involves physical contact. The specific meaning of assault varies between countries, but can refer to an act that causes another to apprehend immediate and personal violence, or in the more limited sense of a threat of violence caused by an immediate show of force. Assault in many US jurisdictions and Scotland is defined more broadly still as any intentional physical contact with another person without their consent; but in England and Wales and in most other common law jurisdictions in the world, this is defined instead as battery. Some jurisdictions have incorporated the definition of civil assault into the definition of the crime making it a criminal assault intentionally to cause another person to apprehend a harmful or offensive contact.
-- Wikipedia
Legal Definition
1. Intentionally putting another person in reasonable apprehension of an imminent harmful or offensive contact. No intent to cause physical injury needs to exist, and no physical injury needs to result. So defined in tort law and the criminal statutes of some states.

2. With the intent to cause physical injury, making another person reasonably apprehend an imminent harmful or offensive contact. Essentially, an attempted battery. So defined in the criminal statutes of some states.

3. With the intent to cause physical injury, actually causing such injury to another person. Essentially, the same as a battery. So defined in the criminal statutes of some states, and so understood in popular usage.
Illustrative caselaw
See, e.g. Raess v. Doescher, 883 N.E.2d 790 (Ind. 2008).
See also
Legal Definition
Crim. law. An assault is any unlawful attempt or offer with force or violence to do a corporal hurt to another, whether from malice or wantonness; for example, by striking at him or even holding up the fist at him in a threatening or insulting manner, or with other circumstances as denote at the time. an intention, coupled with a present ability, of actual violence against his person, as by pointing a weapon at him when he is within reach of it. 6 Rogers Rec: 9. When the injury is actually inflicted, it amounts to a battery. (q. v.)

2. Assaults are either simple or aggravated. 1. A simple assault is one Where there is no intention to do any other injury. This is punished at common law by fine and imprisonment. 2. An aggravated assault is one that has in addition to the bare intention to commit it, another object which is also criminal; for example, if a man should fire a pistol at another and miss him, the former would be guilty of an assault with intent to murder; so an assault with intent to rob a man, or with intent to spoil his clothes, and the like, are aggravated assaults, and they are more severely punished than simple assaults. General references, 1 East, P. C. 406; Bull. N. P. 15; Hawk. P. B. b. 1, c. 62, s. 12; 1 Russ. Cr. 604; 2 Camp. Rep. 650 1 Wheeler's Cr. C. 364; 6 Rogers' Rec. 9; 1 Serg. & Rawle, 347 Bac. Ab. h. t.; Roscoe. Cr. Ev. 210.
-- Bouviers Law Dictionary
Legal Definition
An unlawful attempt or offer, on the part of one man, with force or violence, to inflict a bodily hurt upon another. An attempt or offer to bcat another, without touching him; as if one lifts up his cane or his fist in a threatening manner at another ; or strikes at him, but misses him. 3 Bl. Comm. 120; 3 Steph. Comm. 469.

Aggravated assault is one committed with the intention of committing some additional crime; or one attended with circumstances of peculiar outrage or atrocity. Simple assault is one committed with no intention to do any other injury. An assault is an unlawful attempt, coupled with a present ability, to commit a violent injurv on the person of another. Pen. Code Oal. § 240. An assault is an attempt to commit a violent injury on the person of another. Code Ga. 1882, § 4357.

An assault is any willful and unlawful attempt or offer, with force or violence, to do a corporal hurt to another. Pen. Code Dak. § 305.

An assault is an offer or an attempt to do a corporal injury to another; as by striking at him with the hand, or with a stick, or by shaking the fist at him, or presenting a gun or other weapon within such distance as that a hurt might be given, or drawing a sword and brandishing it in a menacing manner; provided the act is done with intent to do some corporal hurt. United States v. Hand, 2 Wash. C. C. 435, Fed. Cas. No. 15,297.

An assault is an attempt, with force or violence, to do a corporal injury to another, and may consist of any act tending to such corporal injury, accompanied with such circumstances as denote at the time an intention, coupled with the present ability, of using actual violence against the person. Hays v. People, 1 Hill (N. Y.) 351.

An assault is an attempt or offer, with force or violence, to do a corporal hurt to another, whether from malice or wantonness, with such circumstances as denote, at the time, an intention to do it, coupled with a present ability to carrv such intention into effect. Tarver v. Slate, 43 Ala. 354.

An assault is an intentional attempt, by violence, to do an injury to the person of another. It must be intentional; for, if it can be collected, notwithstanding appearances to the contrary, that there is not a present purpose to do an injfury, there is no assault. State v. Davis, 23 N. C. 127, 35 Am. Dec. 735.

In order to constitute an assault there must be something more than a mere menace. There must be violence begun to be executed. But, where there is a clear intent to commit violence, accompanied by acts which if not interrupted, will be followed by personal injury, the violence is commenced and the assault is complete. People v. Yslas, 27 Cal. 633.

Simple assault. An offer or attempt to do bodily harm which falls short of an actual battery ; an offer or attempt to beat another, but without touching him; for example, a bio delivered within striking distance, but which does not reach its mark. See State v. Light-sey, 43 S. (3. ll4, 20 S. E. 975; Norton v. State, 14 Tex. 393.
-- Black's Law Dictionary
Legal Definition
An attempt, or offer, with force and violence, to do corporal hurt to another. See 11 Am. St. Rep. 830.
-- Ballentine's Law Dictionary