In rhetoric, a period is an unusually impressive, well-balanced, and stately sentence. Strictly speaking, a periodic sentence is a sentence whose opening clauses do not express a complete thought until the main clause, which typically comes at the end. The preceding clauses are often many, and exhibit parallel constructions and other rhetorical balancing devices. The overall effect should be of a slow sentence building to a climax. An example from Jeremy Taylor's Holy Living and Holy Dying illustrates how these sentences can be used to great effect:

But so have I seen a Rose newly springing from the clefts of its hood,
  and at first it was fair as the Morning,
and full with the dew of Heaven,
as a Lambs fleece;

but when a ruder breath had forced open its virgin modesty,
  and dismantled its too youthful and unripe retirements,
it began to put on a darknesse,
 and to decline its softnesse,
  and the symptomes of a sickly age;

it bowed the head,

and broke its stalk,
and at night having lost some of its leaves,
and all of its beauty,

it fell into the portion of weeds and outworn faces.

Each of the several clauses is incomplete without the closing clause, which contains the main verb fell. The clauses in the sentence play against one another, reinforcing each other with parallel structures and internal assonance.

A period opens like a collapsible telescope, each phrase developing out of the preceding one:

"Aretino prospered, living from hand to mouth as a hanger-on in the literate circle of his patron, sharpening his satirical talents on the gossip of politics and the papal curia, and turning the coarse Roman pasquinade into a rapier weapon of satire, until his sixteen ribald Sonetti Lussuriosi, written to accompany Giulio Romano's exquisitely beautiful but utterly pornographic drawings, falling into the hands of an engraver, finally lost him the public patronage of Pope Leo X." — Wikipedia, Pietro Aretino

Periodic sentences are common in Greek and Latin writers such as Cicero, who is generally considered to be the Western world's master in this rhetorical device. English writers whose works are famous for their well-crafted periodic sentences include:

• Sir Thomas Browne
• Edward Gibbon
• Samuel Johnson
• Jeremy Taylor
• Thomas de Quincey
• Sir Winston Churchill

Rhetoric