Lowered area in front of a stage where an orchestra accompanies the performers WordNet 3.0 & Open English WordNet

An orchestra pit is the area in a theater in which musicians perform. Wikipedia

A sunken area in front of a stage in which musicians, largely hidden from view, play music to accompany stage performances. Wiktionary

Sunken area in front of a stage. Wiktionary (translation)

Formerly, that part of a theatre, on the floor of the house, below the level of the stage and behind the orchestra; now, in England, commonly the part behind the stalls; in the United States, the parquet; also, the occupants of such a part of a theatre. Wiktionary

Part of a theatre. Wiktionary (translation)