Opera is a musical form delivered through a staged performance that consists of an orchestra, singers, and often dancers. An opera is typically the work of a composer and a librettist (author of opera text). The opera genre was inspired by Neo-Platonic conception of Greek tragedy as a combination of drama and music. Thanks to the use of recitatives, operas typically have uninterrupted music even in between arias.
An opera = symphony + recitative + aria + ensemble + chorus + dance (esp. Italy)