What about "platform" or "system"? Words like that capture the notion that there are several "sub" applications working in tandem to form something broader.
"So, is it an app?" "Not so much an app, as a platform for ______. We're building apps for each mobile OS as well as a web app."
The word "platform" is usually used to refer to the operating system and hardware, but it can also be used more broadly to refer to a base of technology on which other technologies are built. See: http://searchservervirtualization.techtarget.com/definition/platform