Principal Photography
1. Principal photography is the phase of film production in which the movie is filmed, with actors on set and cameras rolling, as distinct from pre-production and post-production.
Principal photography is usually the most expensive phase of film production, generally due to actor, director, and set crew salaries, the costs of certain shots, including any props or on-set special effects. Its start generally marks a point of no return for the financiers, because until it is complete there is unlikely to be enough material filmed to release a final product needed to recoup costs.
Source: Wikipedia