The name amphitheatre (alternatively amphitheater) is given to a public building of the Classical period (being particularly associated with ancient Rome) which was used for spectator sports, games and displays.
Apart from function, the important outward distinction between an amphitheatre and a theatre is that an amphitheatre is round or oval in shape (whereas a theatre is semi_circular). However, an amphitheatre differs from a circus, which was used for racing and looked more like a very long, narrow horse shoe.
The best-known amphitheatre in the world is the Roman Colosseum, which is more correctly termed the Flavian amphitheatre (Amphitheatrum Flavium), after the Flavian dynasty who had it built.
Nijmegen (No longer extant, but traced out in the street paving of the present-day Rembrandstraat (http://www.viamichelin.com/viamichelin/gbr/dyn/controller/mapPerformPage?strAddress=rembrandtstraat&strLocation=nijmegen&strCP=&strCountry=000000285&strStartAddress=&strStartCity=&strStartCP=&strStartCityCountry=000001138&strDestAddress=&strDestCity=&strDestCP=&strDestCityCountry=000001138) with a few bits of its foundations still visible)
In contemporary use, the term amphitheatre is often used to refer to semi-circular, accoustically vibrant performances spaces, particularly those which are located outdoors. These often include standing structures, sometimes curved or "bowl" shaped, both behind the stage and behind the audience, creating an area which echoes or amplifies sound, making the amphitheatre ideal for musical or theatrical performances.
A natural amphitheatre is a performance space located in a spot where a steep mountain or a particular rock formation naturally amplifies or echoes sound, making it ideal for musical and theatrical performances. The term amphitheatre can also be used to describe naturally occurring formations which would be ideal for this purpose, even if no theatre has been constructed there.
article "Amphitheatrum" (http://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA*/Amphitheatrum.html) (Smith's Dictionary of Greek and Roman Antiquities)
Rome: Colosseum at LacusCurtius (http://penelope.uchicago.edu/Thayer/E/Gazetteer/Places/Europe/Italy/Lazio/Roma/Rome/Colosseum/home.html) with Platner article
The Memorial Amphitheater was the dream of Judge Ivory G. Kimball, who wished to have a place to assemble and honor the American defenders.
The Amphitheater is constructed mainly of Vermont-quarried Danby marble.
The Memorial Display Room, between the amphitheater and the Tomb of the Unknowns, houses plaques and other tributes presented in honor of the four service members interred at the Tomb of the Unknowns (first known as the Tomb of the Unknown Soldier).