Actually, activating "unknown sources" in the Android settings is for nothing else but side-loading. While technically there's a difference between activating that configuration option and performing the actual side-loading process, is this really a reason for having separate tags?
I say no. "Unknown sources" is (should be) about the setting. "Sideloading" is (should be) about the act of installing an app outside of the context of the Market/Play Store/etc.
They are related, sure, but I don't think they're the same.