How to define a word

First write:

• It is...

Second: Define the object/person/place using a more general word:

• (Hammer): It is a tool that..
• (School): It is a building that...
• (Blue): It is a colour that...
• (Park): It is a place where...
• (President): It is someone who...
• (Cat): It is an animal which...

Third: If it is an adjective you can say:

• (Empty): It is the opposite of "full".
• (Near): It is the opposite of "distant".

Fourth: Then you can add some more features:

• It consists of...
• It is made of...
• It can be bought in...
• They grow in (Europe).
• People eat it with...
• It is often used for...

EXAMPLES

• (Hammer): It is a tool that Iis made of wood and metal and it is used for striking.
• (Table):  It is a piece of furniture usually supported by one or more legs and that has a flat top surface on which objects can be placed. It is often used for eating or studying. 
• (Doctor): It is a person who is licensed to practise medicine and often works in hospitals or clinics.