In law, unlawful entry onto land. Trespass was formerly defined as wrongful conduct causing injury or loss; today it is generally confined to issues involving real property (see real and personal property). Once a trespass is proved, the trespasser is usually held liable for any damages resulting, regardless of whether the trespasser was negligent or the damage was foreseeable. Criminal trespass, trespass to property that is forbidden by statute, is punishable as a crime.
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Trespass (Fr. trespas, a crime, properly a stepping across, from Lat. trans, across, and passus, step, cf. "transgression," from transgredi, to step across) is a legal concept, which refers to intrusion into another person's property. Most commonly people think of trespass to land, which can be a crime a tort, or both, depending on the jurisdiction. For instance, in England and Wales, despite the prevalence of notices asserting that "trespassers will be prosecuted", unless the trespass is aggravated in some way it is not a crime, and will only be a tort.
There are also torts for trespass to chattels and trespass to the person. In most countries it is classed as an "intentional tort" (you need to intend to have trespassed to be held liable) but in some countries (e.g. Australia) it has been subsumed into the law of negligence.
In England up to 1694 the trespasser was regarded, nominally at any rate, as a criminal, and was liable to a fine for the breach of the peace, commuted for a small sum of money, for which 5 Will. and Mar. C. 12 (1693) substituted a fee of 6s. 8d. recoverable as costs against the defendant. Trespass is not now criminal in England except by special statutory enactment, e.g. the old statutes against forcible entry, the game acts, and the private acts of many railway companies. When, however, trespass is carried sufficiently far it may become criminal, and be prosecuted as assault if to the person, as nuisance if to the land. At one time an important distinction was drawn between trespass general and trespass special or trespass on the case, for which see Tort. The difference between trespass and case was sometimes a very narrow one: the general rule was that where the injury was directly caused by the act of the defendant the proper remedy was trespass, where indirectly case. The difference is illustrated by the action for false imprisonment: if the defendant himself imprisoned the plaintiff the action was trespass; if a third person did so on the information of the defendant it was case. A close parallel is found in Roman law in the actio directa under the lex Aquilia for injury caused directly, the actio utilis for that caused indirectly. One of the reasons for the rapid extension of the action on the case, especially that form of it called assumpsit, was no doubt the fact that in the action on the case the defendant was not allowed to wage his law (see "legal wager").
In its more restricted sense trespass is generally used for entry on land without lawful authority by either a man, his servants or his cattle. To maintain an action for such trespass the plaintiff must have possession of the premises. The quantum of possession necessary to enable him to bring the action is often a question difficult to decide. In most instances the tenant can bring trespass, the reversioner only case. Remedies for trespass are either judicial or extra-judicial. The most minute invasion of private right is trespass, though the damages may be nominal if the injury was trivial. On the other hand, they may be exemplary if circumstances of aggravation were present. Pleading in the old action of trespass was of a very technical nature, but the old-fashioned terms alia enormia, replication de injuria, new assignment, etc, once of such frequent occurrence in the reports, are of merely historical interest since the introduction of a simpler system of pleading, unless in those American states where the old pleading has not been reformed. The venue in trespass was formerly local, in case transitory. In addition to damages for trespass, an injunction may be granted by the court. The principal instances of extra-judicial remedies are distress damage feasant of cattle trespassing, and removal of a trespasser without unnecessary violence, expressed in the terms of Latin pleading by molliter manus imposuit. Trespass may be justified by exercise of a legal right, as to serve the process of the law, or by invitation or license of the owner, or may be excused by accident or inevitable necessity, as deviation from a highway out of repair. Where a man abuses an authority given by the law, his wrongful act relates back to his entry, and he becomes a trespasser ab initio, that is, liable to be treated as a trespasser for the whole time of his being on the land. Mere breach of contract, such as refusal to pay for wine in a tavern which a person has lawfully entered, does not constitute him a trespasser ab initio. A trespass of a permanent nature is called a continuing trespass; such would be the permitting of one's cattle to feed on another's land without authority.
In Scots law trespass is used only for torts to land. By the Trespass (Scotland) Act 1865 trespassers are liable on summary conviction to fine and imprisonment for encamping, lighting fires, &c., on land without the consent and permission of the owner.
Although criminal and civil trespass laws vary from each jurisdiction, the following facets are common:

The term 'trespass' is also used for a transgression in general, also in the traditional version of the Lord's Prayer.
There are many methods land owners use to prevent trespassing, usually depending on the terrain, risk, importance (personal, cultural or economic) and size of the property.
Some of the most common are also the most basic - barbed wire, warning signs and fencing. See also Physical security.