What do you call a word that has the same meaning as another word?

A word that has the same meaning as another word is referred to as a synonym. Synonyms are used in language to convey similar meanings, allowing for more variety and nuance in expression. For instance, the words "happy" and "joyful" function as synonyms because they both convey a similar sense of positive emotion. This can enhance writing and communication by providing alternative ways to express similar concepts.

In contrast, antonyms are words that have opposite meanings, homonyms are words that sound the same but have different meanings, and adjectives are words that describe or modify nouns. None of these terms relates to the concept of having the same meaning, which is why they do not fit the definition presented in the question.