A swing coat is an A-line coat that has been cut full enough that the hem swings when the wearer walks. This is a retro look and I'm not sure that the A-line/"trapèze" description is enough to describe the style since many garments can be A-line without the additional fullness that allows the garment to swing. If you don't feel comfortable using swing, I suggest adding something regarding the retro look (look rétro) to Stephanie Bellumat answer.



On the chest of the coat, the retro fashion style displays between four and eight large buttons, which are arranged in double breasted alignment. This winter coat is often yoked or empire cut and the coat hemline sweeps into **typical sixties A-line swing**.
http://www.fashion-era.com/Coats_history/winter_coats_trends...

Another trend for autumn 2008 is pyramid **coats with a sixties/seventies swing feel**. These swing coats flow like an A-line pyramid from the shoulders and they have matching A-line sleeves. Many dresses follow similar silhouette lines. This trend has been gaining momentum for some time and along with cropped jackets will leave a notable fashion history silhouette for the latter decade.
http://fashion.covermagazin.com/coats.htm

The Swing Coat is a wonderful garment, done in a subtle 1920's Art Déco inspired Jaquard. Cut on the bias for that wonderful swing, with a rich Satin-back Crèpe lining.
http://www.caninecachet.com/Merchant2/merchant.mvc?Screen=CT...

Ça me rappelle un manteau de ma grand mère, qui date des années 1950, **un swing coat** qui n’as également pas de fermeture, et qui est en plus un peu trop grande pour moi…
http://theremina.wordpress.com/2008/08/28/en-attendant-lhive...

Manteau Swing
69,50 $ chez Old Navy
http://www.divine.ca/fr/mode-et-beaute/articles/c_10_i_3433/...


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Note added at 2 days7 hrs (2009-01-06 19:23:22 GMT)
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Correction:
to Stephanie Bellumat'S answer.