Well, in the realest sense, it is both. Sencha almost always refers to the Japanese-medium quality-"daily tea," or "daily Sencha." A step below Gyokuro, and many steps above Bancha, Sencha is a great, full green tea.
However, the Chinese also have a "Sencha." This tea is looked down on so much that it has been dubbed "Chinencha" by TeaChat members* detailing it's inability to match up with the Japanese green Sencha.