Russian Empire
The Russian Empire, also known as Imperial Russia or simply Russia, was an empire that existed across Eurasia and North America from 1721, following the end of the Great Northern War, until the Republic was proclaimed by the Provisional Government that took power after the February Revolution of 1917.
Russian Republic
The term Russian Republic is sometimes used erroneously for the period between the abdication of the Emperor Nicholas II on 2 March 1917 (15 March, N.S) and the declaration of the Republic in September. However, during that period the future status of the monarchy remained to be resolved.