Not really. If you want to be super technical, Russian Imperial Stout is a BJCP category that has the following guidelines:
OG: 1.075 – 1.115
IBUs: 50 – 90 FG: 1.018 – 1.030
SRM: 30 – 40 ABV: 8 – 12%
As well as a strong list of expected flavors/aromas/ingredients and various characteristics.
In practice, it just takes too long to say "Russian Imperial Stout" out loud, so imperial stout is easier. Also, if you feel like making a big stout but don't want to follow the guidelines, you could leave off the russian bit. Or you could keep it in, because RIS is just a name and a big stout is an imperial stout is a Russian Imperial Stout (tm).
Historically, this big stout was invented to be a gift to the Russian royal family and court. For naming, you can use them interchangeably.