I found in Macmillan Dictionary that
homonym: a word that is spelled the same or sounds the same as another word but has a different meaning
homograph: a word that is spelled the same as another word but has a different meaning and sometimes a different pronunciation
According to these definitions, I think a homograph can sometimes be a homonym. Am I right?
Homophones and homographs are the two types of homonym.
Thank you all for your comments. It is interesting to know that there is a clear distinction between homophones and homographs who are children of homonyms.
My plan was to debate on the useless existence of "homograph". But I now think it is fine.
A homonym is a cover term including both homophone and homograph.
A homograph is one of two or more words of different meaning which are spelled in the same way. Homographs may be pronounced the same or differently. Examples: bear (a large animal) and bear (the verb) are homographs which are pronounced identically. While lead (the name of a metal – noun) and lead (the verb) are homographs which are pronounced differently.
A homophone is one of two or more words of different meaning which are pronounced in the same way. Examples are: bear (the name of an animal), bear (the verb) and bare (naked)