Assault is a crime of violence against another person. In some jurisdictions, including Australia and New Zealand assault refers to an act that causes another to apprehend immediate and personal violence, while in other jurisdictions, such as the United States, assault refers only to the threat of violence caused by an immediate show of force. Simple assaults that do not involve any aggravation such as use of a deadly weapon are distinguished from aggravated assaults in some juridictions.
Assault is often defined to include not only violence, but any physical contact with another person without their consent. In Common Law jurisdictions, including England and Wales and the USA, battery is the crime that represents the unlawful physical contact, though this distinction does not exist in all jurisdictions. Exceptions exist to cover unsolicited physical contact which amount to normal social behavior (for example, patting someone on the back): see (in England and Wales) Collins v. Wilcox[1984] 3 All ER 374.
In most jurisdictions, the intention to cause grievous bodily harm (or its equivalent) may amount to the mental requirement to prefer a charge of murder in circumstances where the harm inflicted upon the victim proves fatal. In England and Wales, this fact was criticised by Lord Edmund-Davies in Cunningham [1982] AC 566.

American jurisprudence
Aggravated assault is, in some jurisdictions, a stronger form of assault, usually using a deadly weapon. A person has committed an aggravated assault when that person:
Aggravated assault is usually differentiated from simple assault by the offender's intent (i.e., to murder, to rape etc.), the extent of the injury to the victim, or the use of a deadly weapon, although legal definitions vary between jurisdictions. Sentences for aggravated assault are generally more severe, reflecting the greater degree of harm or malice intended by the perpetrator.

attempts to cause serious bodily injury to another person; or
causes such injury purposely, knowingly, or recklessly in circumstances where the person has exhibited indifference to human life; or
attempts or causes bodily injury to another person with a deadly weapon. Aggravated assault
Although the range and precise application of defenses varies between jurisdictions, the following represents a list of the defenses that may apply to all levels of assault:

General defenses to assaults
Consent may be a complete or partial defense to assault. In some jurisdictions, most notably England, it is not a defense where the degree of injury is severe, as long as there is no legally recognised good reason for the assault.. This can have important consequences when dealing with issues such as consensual sadomasochistic sexual activity, the most notable case being the Operation Spanner case. Legally recognised good reasons for consent include; surgery, activities within the rules of a game (Burnes), bodily adornment (R v Wilson), or horseplay (Jones and others). However, any activity outside the rules of the game is not legally recognised as a defence of consent.

Consent
Police officers and court officials have a general power to use force for the purpose of effecting an arrest or generally carrying out their official duties. Thus, a court officer taking possession of goods under a court order may use force if reasonably necessary. However in Scottish Law, consent is not a defense for assault.

Arrest and other official acts
In some jurisdictions, caning and other forms of corporal punishment are a part of the culture. Evidently, if it is a state-administered punishment, e.g. as in Singapore, the officers who physically administer the punishment have immunity. Some states also permit the use of less severe punishment for children in school and at home by parents. In English law, s58 Children Act 2004, limits the availability of the lawful correction defense to common assault under s39 Criminal Justice Act 1988.

Punishment
Self defense and defense of others may be defenses to liability. They usually require that force was necessary and the degree of force was reasonable.

Self-defense
This may or may not involve self defense in that, using a reasonable degree of force to prevent another from committing a crime could involve preventing an assault, but it could be preventing a crime not involving the use of personal violence.

Prevention of crime
Some states allow force to be used in defense of property, to prevent damage either in its own right, or under one or both of the preceding classes of defense in that a threat or attempt to damage property might be considered a crime (in English law, under s5 Criminal Damage Act 1971 it may be argued that the defendant has a lawful excuse to damaging property during the defense and a defense under s3 Criminal Law Act 1967) subject to the need to deter vigilantes and excessive self-help.

Variations of Assault in England and Wales

Affray
Battery (crime)
Assault (tort)
Street fighting
Domestic violence
Gay-bashing
Hate crime
Mayhem
Misdemeanor
Offences Against The Person Act 1861
Terroristic Threats