Most Westerners would say that when Palestinian Arabs refer to the "nakba", or catastrophe, that they are referring to the dispossession of hundreds of thousands of Palestinian Arabs from their homes in 1948, and their defeat in the 1948 war. Certainly pro-Palestinian Arab Westerners use the term that way, and Palestinian Arabs speaking to Westerners seem to keep that definition as well.
Wikipedia:Nakba Day, meaning "Day of the catastrophe" is a annual day of commemoration for the Palestinian people of their displacement and dispossession as a result of their defeat in the 1948 Palestine war.
Electronic Intifada: Every year Palestinians commemorate the Nakba ("the catastrophe"): the expulsion and dispossession of hundreds of thousands Palestinians from their homes and land in 1948.
Nakba Archive: During the 1948 war with the nascent state of Israel it is estimated that around half of the 1.4 million Palestinian Arabs were driven from their homes or fled, to neighboring Arab states. This period of Palestinian history has come to be known as al-Nakba, ‘the catastrophe’.
To the West, this makes sense - it can certainly be seen as catastrophic that a large group of people become homeless in the space of a year, no matter the circumstances.
There is another definition of Nakba, however, one that Westerners do not see nearly as much.
Palestine News Network: The Israelis are gearing up to celebrate 60 years since the inception of their state, what the Palestinians refer to as the Catastrophe, Al Nakba.
Gulf News: Not quite two weeks from now, on May 8, Palestinians will commemorate the Nakba, when their homeland was dismembered exactly 60 years ago that day.
In other words, to Arabs, the Nakba is more associated with the establishment of Israel than with any negative events that occurred to Arabs in Palestine.
A little reflection shows that the idea that the Nakba is meant to show solidarity with Palestinian Arabs and not just antipathy to Zionist Jews is ludicrous. After all, the Palestinian Arabs have been kept in stateless limbo due to the direct actions of their Arab brethren and their own failed leaders, who cynically use them as pawns - to pressure Israel.
It is most instructive that "Nakba Day" is timed to coincide with the anniversary of Israel's independence, not with the anniversary of any notable acts of dispossession or massacres like Deir Yassin. The true catastrophe, in Arab thought, is the creation of a Jewish state and not the tragedies that happened to the Arab citizens who fled or died.
Palestinian Arabs cannot even conceive that there is a difference between the two concepts; that Israel's establishment was not meant to displace hundreds of thousands of people. They cannot imagine that the Jews at the time were far more interested in surviving and in building a viable state where they could live in peace than in hurting others - to Arabs of Palestinian descent, force-fed a steady diet of lies and propaganda, the Jews' entire purpose was destructive and not positive. (This is, of course, another aspect of their own projection of their desires vis a vis the Jews of Palestine in 1948.)
But even deeper is the idea that Jews establishing a tiny state on their historic homeland itself is what they consider their disaster - even if not one Arab had left their home they would still regard Israel's Independence Day to be their catastrophe.
And they still do.