A fixed form consisting of fourteen lines of 5-foot iambic verse. In the English or Shakespearean sonnet, the lines are grouped in three quatrains (with six alternating rhymes) followed by a detached rhymed couplet which is usually epigrammatic. In the original Italian form, such as Longfellow's " Divina Commedia," the fourteen lines are divided into an octave of two rhyme-sounds arranged abba abba and a sestet of two additional rhyme sounds which may be variously arranged. This latter form tends to divide the thought into two opposing or complementary phases of the same idea.
Sidelight: A variant of the Shakespearean form is the Spenserian sonnet which links the quatrains with a chain or interlocked rhyme scheme, abab bcbc cdcd ee.
Sidelight: The English language contains fewer rhyming possibilities than Italian, so the Shakespearean adaptation relieved English poets from the greater difficulty of rhyming in the Italian sonnet format.
Sidelight: A sonnet sequence is a series of sonnets in which there is a discernable unifying theme, while each one retains its own structural independence. All of Shakespeare's sonnets, for example, were part of a sequence,
(See Quatorzain, Volta)
(See also Anthology, Canon, Companion Poem, Cycle, Lyric Sequence)

This definition is in context to Poetry. See more contextual defintions for Sonnet.