Magnum opus is a Latin phrase meaning a writer's greatest or most important work. In poetry, it usually refers to the single poem, collection, or long poem that best represents a poet's achievement or ambition.
The phrase can be used broadly for any artist, but in literary discussion it often points to the work a poet is best known for or most likely to be remembered by.
Example
Many readers consider Whitman's Leaves of Grass his magnum opus,
where his voice, themes, and style come together in full.