The word boundary has no special meaning in law. In land ownership, it is understood in two ways:

the legal boundary – is the line which is not visible on the ground which divides one person’s land from another’s. It is an exact line having no thickness. It is rarely identified with any precision either on the ground or in the deeds.

the physical boundary – is a physical feature such as a fence, wall or a hedge. 

The legal boundary may be intended to follow the physical boundary, but this is not always the case. For example, the legal boundary may run down the centre of a hedge, along either of its sides, or anywhere within or beyond it.