“The act of remaining or entering on land without a right to do so”
Trespass is an act which is not a criminal offence in the UK (at least at the moment) and instead is dealt with under civil law. Usually this means that one person will take another to court and attempt to collect “damages” (money) as compensation, an order for possession of the land (if the other is squatting such land) or an injunction (an order prohibiting certain activities, such as entering a specific piece of land or property).
Trespassing in itself is not illegal, unless you are trying to disrupt a lawful activity (see “aggravated trespass”) or unless it concerns an animal research organisation (see “SOCPA s145”). If you refuse to leave the land when asked by a land-owner or agent of the land-owner, the police may be called. The land-owner or agents can use reasonable force to remove you from the land. Quite often, people are arrested to prevent a breach of the peace simply for refusing to leave land, the police saying that they are provoking the landowner to use violence against them. If you resist the police when they remove you, you may be arrested for obstructing a police officer.