This situation describes the existence of a 'property' right. A property right can be defined as a legal right that exists between a person and a 'thing'. In the example, the person is A, and the thing is the boat. A is described as the 'owner' of the boat, and ownership is one of the most familiar rights that can exist over property.
The law protects property rights by giving the holder of the right a legal action against anyone who wrongly interferes with it.
There are other property rights besides ownership: these will be discussed later.
You should note that the Latin word for 'thing' is 'res' and this explains why property rights are sometimes referred to as 'real' rights - legal rights that exist in relation to a 'res'.
The French word for 'thing' is 'chose' and this word has also found its way into Australian property law, via English law. For example, some kinds of thing can be referred to as a 'chose in possession'.