Sinfonia is the Italian word for symphony. In English it most commonly refers to a 17th- or 18th-century orchestral piece used as an introduction, interlude, or postlude to an opera, oratorio, cantata, or suite. It has also sometimes been used for other types of music (see below).
Late Renaissance - Early Baroque
In the very late Renaissance and early Baroque, a sinfonia was an alternative name for a canzona, fantasia or ricercar. These were almost always instrumental forms, all rooted however in a polyphonic tradition. Later in the Baroque period it was more likely to be a type of sonata, especially a trio sonata or one for larger ensemble. Still later in the Baroque era, the word was used to designate an instrumental prelude, as described in the next section.
Overture or early symphony
In larger vocal-instrumental forms of the 17th and 18th centuries, for example operas and oratorios, a sinfonia was generally an instrumental prelude, sometimes also an interlude/intermezzo or postlude, providing contrast with adjacent vocal or otherwise different sections.
A specific form of such kind of preluding piece, in the early 18th century, was the three-movement sinfonia which became the standard type of overture to an Italian opera. Most of the time these pieces were in D major (for maximizing open-string resonance on string instruments), opening and ending with a fast movement, with a slow movement in the middle. Examples of this type of Italian sinfonia are the numerous three-movement opera overtures by Alessandro Scarlatti, all archetypical Italian overtures.
In France however overtures had always been one-movement preluding pieces, usually in a A-B-A form, where the "A" sections had a slow tempo with a stately (double)dotted rhythm, while the "B" middle section was comparatively fluent and fast. This musical form became known as the French overture. By the time this type of overture was adapted by German composers like Bach and Handel from the early 18th century on, it could be as well the preluding movement of a (dance) suite, in which case overture was sometimes used as a synonym for the entire suite (e.g. Bach's French Overture, BWV 831).
Most of Handel's operas and oratorios start with the French type of overture movement, even if he occasionally calls such movement a sinfonia (as he did for the Messiah, actually calling it a "Sinfony" [ audio (help·info)]). But Handel would use the Italian type of orchestral prelude/interlude too, for instance the Introduzione to the cantate Delirio amoroso, HWV 99. Also the instrumental Pifa featuring in the Messiah did not so much derive from French examples. An interesting anecdote is that when Mozart made a German version of the Messiah, some 30 years after Handel's death, he changed the name of the opening Sinfony to Ouvertüre, but more or less did away with its French characteristics: he softened the dotted rhythm of the "A" section with some more flowing horn melodies, and by speeding it up a bit also made it less distinct from the "B" section: the result is that the "A" part appears as not much more than a moderate preamble to a "fast" symphonic movement (the "B" section).
Meanwhile, also from the early 18th century on, the 3-movement Italian type of sinfonia had started to lead a life of its own: it could be composed as an independent concerto-like piece (without soloists however). For instance Vivaldi composed 3-movement independent sinfonias, not so different from some of his string concertos, as well as composing similar sinfonia preludes for his operas.
Bach sometimes used the term sinfonia in the then-antiquated meaning of an instrumental single-movement piece, e.g. for the keyboard Inventions and Sinfonias BWV 787-801, using a three-voice polyphonic style. Note that in 20th century, publishers started to publish these sinfonias as "Three-Part Inventions", where "Part" is an independently flowing melody ("voice", but in the instrumental meaning) in a single-movement work.
If Bach opened a vocal work with one or more separate instrumental movements (which was all in all not so often), he would usually call such piece a sinfonia or alternatively a sonata. For the sinfonias the style would be rather Italian (also for the single-movement ones) than French:
One-movement sinfonia opening the secular cantatas Non sa che sia dolore, BWV 209 and Mer Hahn en neue Oberkeet, BWV 212
Sinfonia followed by an "adagio" opening the Easter Oratorio, BWV 249. Although the chorus joins in the third movement of that oratorio, these three successive opening movements could be seen as a 3-movement "Italian" sinfonia to the oratorio.
Some opening movements of his church cantatas were like up-beat movements of organ concertos (BWV 29, 35, 49, 169) - later Bach would rework some of these sinfonias to harpsichord concerto movements.
Both Handel and Bach used the French type of overture to start their orchestral suites. For suites they composed for a solo instrument there often was no prelude movement. If there was, that opening movement would usually be either an Overture/Ouverture (in that case always referring to the French style), or otherwise a Prelude/Praeludium. The style of such preludes was less defined but would often emulate the style of a fast movement of an Italian sinfonia.
As the 18th century progressed, the usual name for an instrumental prelude to a vocal/theatrical work would settle on overture. Although such overtures would generally be one-movement pieces, they were no longer in the French style, but rather adapted the Italian preluding sinfonia, for instance a loud, triadic, motto-type leading motif, a reprise preceded with minimal thematic development, and an overall mood of expectation rather than resolution.
The idea of the Italian 3-movement sinfonia as an independent orchestral composition lived on too: the earliest symphonies of Haydn and Mozart were composed in this format. Mozart also composed divertimentos in the Italian sinfonia format, with some ambiguity whether such divertimentos were indeed intended as independent instrumental compositions, or rather as instrumental interludes (for theatre productions etc).
But then Haydn made the Italian sinfonia/non-solistic concerto and the French type of overture/suite meet again: he took the three movements of a sinfonia, and inserted a fourth between the two last movements of the Italian model. That additional movement was a menuet, which had until then only been an almost obligatory movement of a suite. He also took some characteristics of the French style overture movement, as well as of what was the sonata in those days, amongst others the possibility to start the first movement of such a four-movement composition with a slow introductory passage. But the resulting composition was no longer called a "sinfonia" (at least not outside Italy and Spain): the symphony was born.
Symphony with an alternative scope
Later sinfonia would occasionally be used as an alternative name for a symphony, from the Romantic era on. Often, but not always, the title "sinfonia" is used when the work is seen as, or intended to be, lighter, shorter, or more Italianate or Baroquish in character than a full-blown (romantic) symphony (with its dominantly Germanic pedigree).
Examples of such "sinfonias" composed after the classical era include:
Felix Mendelssohn's twelve early symphonies, most of them string symphonies in three movements and all of them composed before his five other more elaborate symphonies, are sometimes called "sinfonias", to distinguish them from the Symphonies 1 to 5 that were published during - or shortly after - the composer's lifetime. The Italian is a composition of the latter series, so always called a "symphony". On the other hand Mendelssohn used the term sinfonia in the "overture" meaning for the first movement of his Lobgesang symphony. This can be seen as one of the many Bach reminiscences Mendelssohn inserts in his music: these references to the old master were especially thick in this "symphony-cantata", as it was to be premiered in the Thomaskirche in Leipzig.
Vincent d'Indy wrote a Sinfonia brevis de bello Gallico that is: "Brief sinfonia of the War in Gaul".
Richard Strauss chose the name Sinfonia Domestica ("Domestic Symphony") for a full scale symphony he composed 1902–1903. Maybe this symphony shows a somewhat sunnier side than most of his other orchestral compositions - but then large parts of the work also portray domestic tiffs and other tensions, ending in an elaborate fugue restoring coherence in the household.
Benjamin Britten wrote a Sinfonia da Requiem in 1941. Here Sinfonia is rather an allusion to seriousness or solemnity, than to any kind of lightness.
^ This definition is taken from the Oxford American Dictionary, which gives the origin of the word as Italian. It is also found in other related languages such as Spanish or Portuguese. For a detailed etymology see Symphony. See also Talk:Sinfonia.
Manfred Bukofzer, Music in the Baroque Era. New York, W.W. Norton & Co., 1947. ISBN 0-393-09745-5
The New Harvard Dictionary of Music, ed. Don Randel. Cambridge, Massachusetts, Harvard University Press, 1986. ISBN 0-674-61525-5
Article Sinfonia, in The New Grove Dictionary of Music and Musicians, ed. Stanley Sadie. 20 vol. London, Macmillan Publishers Ltd., 1980. ISBN 1-56159-174-2