Technique
Theme
From Greek thema: a proposition set down. New Critics distinguished theme from 'message' (a didactic reduction); modern critics regard theme as emergent from structure and language rather than explicitly stated.
Definition
The central idea, moral concern, or insight that a poem explores — distinct from its literal subject or story.
Example
A poem about a storm may carry themes of human helplessness, nature's indifference, the sublime, or mortality.