Organic form is a poem's shape and structure as they grow naturally out of its subject, tone, and movement rather than from an imposed pattern. The line breaks, rhythm, and stanza divisions seem to arise from what the poem is saying and feeling, so the form feels discovered instead of forced.
The term is often used in contrast to fixed or mechanical forms such as sonnets or strict stanza patterns, though a poem can still use tradition and remain organic if its structure fits its content closely.
Example
The porch light flickers.
A moth keeps circling it.
I write in short, uneven lines
because the night keeps stopping,
then starting again.