The so-called Federation of Northern Syria was proclaimed by the regional parliament on March 16th, 2016. Prior to that it was mostly called Rojava (which means west in Kurdish). The new name is a reference to the fact that the Kurdish population was separated between four countries: Turkey, Iran, Iraq and Syria. The regions the Kurds inhabit are respectively called: Bakur, Rojhalat, Bashur and Rojava.
The change was dictated by the territorial expansion of the Syrian Democratic Forces which is the name of the Kurdish and Arab militias supported by the United States. The conquered territories are ethnically diverse. Due to that fact, a new, more universal name was chosen for the area that used to be called Rojava.