A pest is any living organism, whether animal, plant or fungus, which is invasive or troublesome to plants or animals, human or human concerns, livestock, or human structures.
It is a loose concept, as an organism can be a pest in one setting but beneficial, domesticated or acceptable in another.
The Prevention of Damage by Pests Act 1949
The Weeds Act 1959
The Ragwort Control Act 2003