Meaning of "Fitnah" 

The meaning of the word Fitnah, as claimed by Scholar Ibn Kathir in his tafsir, is "Shirk".

and also Muhsin Khan clarifies, "And fight them until there is no more Fitnah (disbelief and worshipping of others along with Allah) and (all and every kind of) worship is for Allah (Alone). But if they cease, let there be no transgression except against Az-Zalimun (the polytheists, and wrong-doers, etc.) (Sura 2:193)ref"

Source: tafsir Ibn kathir (verse 2:193)