Eevery sin is infact, Haq talfi. In this sense, Ithm origianlly means usurption. This is a philosophical epistimology, may not find in the dictionary. Every sin can be traced to its sessence of Haq talfi.
Ithm includes all sins. When it comes in comparison to Udwaan, it would mean usurption, as these two words come as pair. In verse 7:33, the word Udwaan is replaced with Baghi, hence leaving the meaning of usurpation for the word ithm.
There are five kinds of Ithm mentioned in the verse 7:33. Fahsah (sins regarding to sex), Shirk, , Innovation in Deen (Bidah), Transgression and Ithm. now the only sin left is usurption if four others are already extracted from it. This extraction or substration is called Tajreed: The word shuns its meanings to other words of similar meanings when used along with it, and it is reduced to the meaning left over.