A simple way to define a noun is to simply indicate what makes the noun different from its hypernym.

For example, you can get a good definition of "dog" by simply indicating that it is a type of its hypernym--domestic animal--and then indicating what makes a "dog" different from all other domestic animals. A way of showing how a noun is different from others is to describe what it does or what it is used for. So, to define a dog in English, it is perfectly acceptable to say:

a dog is a domestic animal.