This term is bandied about more than a pair of nickers in a knocking shop.
Please excuse the allegory :-)
The term originally meant 'racism against Jewish people' - now it seems to have morphed into meaning something completely different or a range of (sometimes) conflicting meanings.
OK let's define what it is not: It does not now seem to denote a non Jewish person being racist, as people now use the term against Jewish people?
I think a lot of confusion comes about because some people equate criticizing Israel policies (or some of them) as anti-semitism.
Further some people have equated anti-zionism with anti-semitism?
Again I think the confusion in this comes from the dual meaning of the term 'Zionism', some people understand it as a term meaning 'Israel's right to exist as a nation and entity' and some people understand the term to mean 'Colonialism by Israel in expanding it's territory into its neighbors territory', and yet another (more conspiratory) meaning is 'Jewish people trying to impose their way onto other nations and global stage and control them'.
If we concentrate on the first two definitions it's clear that they are completely different things, hence why confusion seems to ensue when someone is called an 'anti-semite' for challenging Israel's policies in some areas.
What is worrying is many diaspora (& Israeli's) to have the freedom to be objective and openly criticize (not Israel's right to exist) but the way it goes about some of its business. Which is later pounced on as 'anti-semitism'.
If a nation of people close their eyes, ears, or mouth - and give a nation free reign without accountability then that can set up an unhealthy (and even dangerous) environment for itself and its neighbors.
And as we know with the cry wolf story if you overuse and abuse the term, then when it is applicable people won't take 'it' or you seriously.