A synonym is a word or phrase that has the same or nearly the same meaning as another word or phrase. Poets may choose among synonyms to find the best tone, sound, rhythm, or level of precision for a line.
Although synonyms overlap in meaning, they are rarely identical in every shade of feeling or use, so a poet's choice can change the mood of a poem.
Example
The moon was bright, the moon was clear,
The moon was pale and calm and near.
Here, bright, clear, pale, and calm work as close synonyms or near-synonyms, each giving the line a slightly different feel.
Related Definitions
[n] two words that can be interchanged in a context are said to be synonymous relative to that context