A Terza Rima is a type of poem which is made up of three-line stanzas linked to each other by rhyme. Each stanza, or tercet, introduces a new rhyme in its middle line, while the first and third lines continue a rhyme from the previous stanza. So the rhyme scheme is going to be A-B-A, B-C-B, C-D-C, and so on.
Typically a terza rima poem ends with a single line that repeats the rhyme of the middle line from the last tercet, or a couplet with two lines that both use the last middle line's rhyme: B-C-B, C-D-C, D, or DD.
The Italian term, literally "third rhyme", indicates that this form is much more widespread in Italian, a language where rhymes are more plentiful than in English. Dante introduced the form in the 14th century, and his Divine Comedy and Inferno are the most celebrated examples of terza rima. English poets have used it successfully, however, including Chaucer, Milton, Shelley and T.S. Eliot.
Example
I have been one acquainted with the night.
I have walked out in rain—and back in rain.
I have outwalked the furthest city light.
I have looked down the saddest city lane.
I have passed by the watchman on his beat
And dropped my eyes, unwilling to explain.
I have stood still and stopped the sound of feet
When far away an interrupted cry
Came over houses from another street,
But not to call me back or say good-bye;
And further still at an unearthly height,
One luminary clock against the sky
Proclaimed the time was neither wrong nor right.
I have been one acquainted with the night.
O wild West Wind, thou breath of Autumn's being,
Thou from whose unseen presence the leaves dead
Are driven, like ghosts from an enchanter fleeing,
Yellow, and black, and pale, and hectic red,
Pestilence-stricken multitudes: O thou,
Who chariotest to their dark wintery bed
The winged seeds, where they lie cold and low,
Each like a corpse within its grave, until
Thine azure sister of the Spring shall blow
Her clarion o'er the dreaming earth, and fill
(Driving sweet buds like flocks to feed in air)
With living hues and odours plain and hill:
Wild Spirit, which art moving everywhere;
Destroyer and preserver; hear, oh, hear!