Something that does more than one thing, and does more than one thing "well." Not a niche app, which have their place. It's often an ecosystem; other apps or websites have instructions telling you to install another app first.
Evernote, OneNote, OfficeMobile, are all "big App".<p>They are "Suite sized" apps for "off desktop" platforms (tablets, phones, TVs, Consoles).<p>A big app is typically productivity software. Typically not something built by 3 guys, but rather a team. Typically not free.