What is Trespasser?

Legal Definition
In the law of tort, property, and criminal law a trespasser is a person who commits the act of trespassing on a property, that is, without the permission of the owner. Being present on land as a trespasser thereto creates liability in the trespasser, so long as the trespass is intentional. At the same time, the status of a visitor as a trespasser (as opposed to an invitee or a licensee) defines the legal rights of the visitor if they are injured due to the negligence of the property owner.
-- Wikipedia
Legal Definition
One who couimits a trespass.

2. A man is a trespasser by his own direct actohen he acts without any excuse; or he may be a trespasser in the execution of a legal process in an illegal manner; 1 Chit. Pl. 183: 2 John. Cas. 27; or when the court has no juris4iction over the subject-matter when the court has jurisdiction but the proceeding is defective and void; when the process has been misapplied, as, when the defendant has taken A's goods on an execution against B; when the process has been abused 1 Chit. Pl. 183-187 in all these cases a man is a trespasser ab initio. And a person capable of giving his assent may become a trespasser, by an act subsequent to the tort. If, for example, a an take possession of land for the use of another, the latter may afterwards recognize and adopt the act; by so doing, he places himself in the situation of one who had previously commanded it, and consequently is himself a trespasser, if the other had no right to enter, nor he to command the entry. 4 Inst. 317; Ham. N. P. 215. Vide 1 Rawle's R. 121.
-- Bouviers Law Dictionary
Legal Definition
One who has committed trespass; one who unlawfully enters or intrudes upon another's land or unlawfully and forcibly takes another's personal property.
See also
-- Black's Law Dictionary