The Empire of Japan covers that part of the History of Japan in which Japan sought to expand, principally by military force, beyond her traditional home islands. At its most basic, it would cover the period beginning with the Meiji Restoration of 1868, and stop with the Surrender of Japan after World War Two in the Pacific. More subtle starting dates might be after the Perry Expedition to Japan in 1859, followed by the acceptance of foreign military training during the late Tokugawa Shogunate. It does not include the postwar growth of Japan into a great economic power.
Essentially, though, it covers the reign of the Meiji (personal name Mutsohito, 1868-1912), Taisho (personal name Yoshihito) and Showa (personal name Hirohito, 1926-1945) Emperors of Japan.