When two or more words mean almost the same thing, they are called antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms.  For example:
hard = difficult (although hard is more antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms)
When we tell our students that two words 'mean the antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms' we need to be careful to add 'in this case'!
When two words are opposite in meaning, they are called antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms. 
The words 'brother' and 'sister' are antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms antonyms because you can't have one without the other.
Some words can include the meaning of other words.  When the word 'vehicle' is used to cover the meanings of 'car', 'bus', 'tram', 'train', 'motorcycle' etc., it is called superordinate or antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms and the words 'car', 'bus', 'tram', 'train', 'motorcycle' etc. are the antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms.
antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms are words which are spelled the same and pronounced the same but have different meanings.
antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms sound exactly the same but are spelled differently and have different meanings.
antonymsconverseHomographsHomonymsHomophoneshypernymhyponymsinformalsamesynonyms are spelled the same but pronounced differently.