I think "sideloading" is a completely ok term. it says what it is,, you load an app, not with the intended method (appstores) but from an alternative source, as

I think "sideloading" is a completely ok term. it says what it is,, you load an app, not with the intended method (appstores) but from an alternative source, aside the official store. not a native English speaker, so I might not fully understand the possible connotations with this particular word, but I do not personally see anything wrong with it.