You can have different strategies for arriving at a definition that is fit for your purpose.

Intensive definition: Describes characteristics

An intensive definition describes characteristics of things:

An intensive definition classifies things. An elegant strategy is to define “special cases” by leaving the explanation of the more general case to another definition:

The following classify, rather than define, individual concepts as being instances of a noun concept:

Extensive definition: Enumerates instances

An extensive definition enumerates all the instances that are meant by the term you are defining:

This definition is great in the sense that it leaves no doubt about whether a given island is or is not a Wadden Island. On the downside, it does not help a reader grasp the idea of a Wadden Island.

Instead of commas, you can enumerate using a bulleted list.

This definition is a closed enumeration. All the instances are mentioned. This makes the definition formal. Sometimes you want to just give a number of examples and you cannot or do not want to mention them all. This is an open enumeration. To indicate that there are more instances, use an ellipsis symbol (three dots, ...) at the end.

Operative definition: Instructs

An operative definition gives instructions. It tells you what you need to do to understand the meaning of the term:

Ostensive definition: Shows

An ostensive definition shows, rather than describes, the meaning of a term.