main theater of war; principal battlefield
Refers to the primary location where a war or major battle is fought. Often used in historical or military contexts.
This region was the main theater of war during World War II.
main market; principal area of competition
Figurative extension: the most important market or competitive arena in business, politics, or other fields.
The smartphone market has become the principal area of competition for companies.
Compound of 主 (shu, 'main') + 戦場 (senjō, 'battlefield'). The figurative business sense is a modern extension.