All the relevant definitions of apartheid use specific language that it is a crime of racial discrimination. The International Convention on the Suppression and Punishment of the Crime of Apartheid says "For the purpose of the present Convention, the term 'the crime of apartheid', which shall
include similar policies and practices of racial segregation and discrimination as practised in
southern Africa, shall apply to the following inhuman acts committed for the purpose of
establishing and maintaining domination by one racial group of persons over any other racial
group of persons and systematically oppressing them" and then goes on to give lots of examples always using the term "racial group."
The Rome Statute defines apartheid as "inhumane acts of a character similar to those referred to in paragraph 1, committed in the context of an institutionalized regime of systematic oppression and domination by one racial group over any other racial group or groups and committed with the intention of maintaining that regime."
The April HRW report accusing Israel of apartheid says that "racial group" doesn't really mean racial group. They have no proof from the source materials, for which the definition of "racial group" was obvious enough not to be defined. Since the actual conventions didn't define the term, HRW took the definition from a completely different Convention.
The International Convention on the Elimination of All Forms of Racial Discrimination (ICERD), which was adopted in 1965 and came into legal force in 1969, defines “racial discrimination” as “any distinction, exclusion, restriction or preference based on race, colour, descent or national or ethnic origin which has the purpose or effect of nullifying or impairing the recognition, enjoyment or exercise, on an equal footing, of human rights and fundamental freedoms in the political, economic, social, cultural or any other field of public life.” The Committee on the Elimination of All Forms of Racial Discrimination (CERD), the UN body charged with monitoring the implementation of the ICERD, has consistently found that members of racial and ethnic groups, as well as groups defined based on descent or their national origin, face racial discrimination.[47] Rather than treat race as constituting only genetic traits, Human Rights Watch uses this broader definition.
On first glance, that sounds like a pretty good argument for an expansive definition of racial discrimination, although perhaps not for a definition of a racial group. (I think an argument can be made that the Apartheid Convention and the Rome Statute intended the narrower definition, and that the expansive definition is explicitly only meant for ICERD ["In this Convention, the term 'racial discrimination' shall mean..."]. Others have argued that the definition of apartheid is specifically based on race alone. But let's set that aside for now. )
Once HRW is relying on ICERD to define what racial discrimination is, they must then include the very next paragraph in ICERD, which applies directly to Israel - and which they do not quote in their report.
This Convention shall not apply to distinctions, exclusions, restrictions or preferences made by a State Party to this Convention between citizens and non-citizens.
This one paragraph completely destroys HRW's "apartheid" argument.
Israeli laws do not distinguish between Israeli Jewish citizens and Israeli Arab citizens. They distinguish between Israeli citizens and non-citizens - which every nation on Earth does.
HRW and others will base their "apartheid" arguments on claims like saying that Jewish "settlers" in the territories have different laws than their Arab neighbors. HRW says that Israeli "policies include limiting the population and political power of Palestinians, granting the right to vote only to Palestinians who live within the borders of Israel as they existed from 1948 to June 1967." But that is a lie - there are thousands of Israeli Arab citizens who live across the Green Line in French Hill, Beit Hanina, Beit Safafa and other communities, who can vote in Israeli elections, just like Israeli Jewish "settlers" can.
And if someone like, say, Peter Beinart decided to move to Ramallah to prove that Palestinians are wonderful people who wouldn't murder him, he would not be allowed to vote in Israeli elections even though he is a Jew - because he is not an Israeli citizen.
Virtually every example of discrimination in the HRW report, as well as in other articles that make the claim of "apartheid," is based not on whether someone is Arab or Jewish, but on whether they are citizens or non-citizens - the exact distinction that the International Convention on the Elimination of All Forms of Racial Discrimination made clear is not to be considered racial discrimination.
This one paragraph in the ICERD demolishes their entire 213 page report.
The authors of the Human Rights Watch report definitely knew this when they decided not to quote the other section of the ICERD that they base their entire argument on.
If this was an issue of conflicting legal arguments, then one could let the lawyers argue it out. But if it was a legal argument, one would assume good will from both sides, pointing out their own interpretations of the law. By quoting only the definitions that agree with HRW's pre-determination of Israeli "apartheid" and pretending that the definitions that disprove the argument don't even exist, HRW shows that its own arguments weren't based on the law to begin with, but on a bastardization of international law meant to attack only one state.