An antonym is a word which means the opposite to another word.

Example
'Sad' is an antonym of ‘happy'.

In the classroom
Antonyms are an interesting way for learners to explore the meaning of words, for example when there are various antonyms for a word with multiple meanings, such as ‘rich' or ‘hard'. This can be done through activities such as mini-presentations of all the antonyms of a word, or by matching pairs.

Further links:

https://www.teachingenglish.org.uk/article/using-opposites

https://www.teachingenglish.org.uk/article/word-building

https://www.teachingenglish.org.uk/article/same-opposite-or-different-dictation