Synonyms are words that mean the same or almost the same. Although many words in English mean similar things, few are true synonyms. Aspects that make words different include register, connotation and collocation.

Example
Synonyms of ‘thin' include ‘slim', ‘slender' and ‘skinny'.

In the classroom
Practice of synonyms includes activities such as ‘odd one out', gap-fill exercises, mapping words on spider-grams, and transforming texts from formal to informal register.