place or venue for (mostly) outdoor sports, concerts, or other events
A stadium is a building in which lots of people may watch an event. They are often used for football and athletics. Some stadiums are paid to add a company name to the name of their stadium. For example, Comerica Park in Detroit, Michigan, or the ALLTEL Stadium in Jacksonville, Florida. When people do this the company has the naming rights to the stadium. The given name of a stadium can change when another company buys the naming rights to that stadium.
An arena is a small indoor stadium. Arenas are often used for many different types of events. These include sports and music.