Technique
Persona
From Latin persona: mask (as worn by Roman actors). The distinction between poet and speaker was theorised by New Critics and is fundamental to modern poetic analysis.
Definition
The speaker or 'mask' adopted by the poet — a voice constructed within the poem, distinct from the biographical author.
Example
Browning's murderous Duke in 'My Last Duchess' is a persona, not Browning; Eliot's Prufrock is not Eliot.