As far I understand, for something to be considered a genocide, there has to be a specific intent to wipe out a class, religion, ethnicity or culture- whether it be planned or spontaneous, with political activism being an exception to that rule- as there's a preexisting term called purge, and we don't treat the purges of the USSR as genocide, which surely had the numbers to match.
But after having read about the Cambodian genocide, it doesn't seem as if there wasn't an intent to wipe any particular target group out. The Khmer Rouge targeted everyone, whether it be buddhists or christians or intellectuals or minorities, as long as they didn't follow their programme.
Of course, there was a large death toll. But sheer numbers isn't the standard by which we discern whether something is a genocide or not, and I can't see why the events in Cambodia aren't called the Cambodian massacre, or even just the Killing fields, but specifically Cambodian genocide.
When most people refer to the Cambodian genocide, they are thinking of the crimes against humanity and war crimes perpetrated by Democratic Kampuchea (Khmer Rouge) against the Khmer people. But, the regime was also guilty of genocide in its legal definition.
DK leaders were accused and convicted of both crimes against humanity, war crimes in violation of the 1949 Geneva accords, and genocide, as well as Cambodia's domestic laws on murder, torture, etc. If you go through individual convictions you can see that not all have been convicted of genocide. For example, one of the most infamous leaders, Ieng Sary, was only charged with genocide as it pertained to the Cham and Vietnamese. His wife, Ieng Thirith, was only charged with genocide in the case of Vietnamese. A top security services official, Kaing Guek Eav, was charged with a rather long list of crimes against humanity and war crimes, but not genocide.
The genocide specifically relates to their treatment of minority populations like the Cham and Vietnamese, though the Thai, Lao, and Chinese also were targeted, as well as the Buddhist clergy/Sangha. If you can look at the table on p. 193 of Genocide: Conceptual and Historical Dimensions you can see for example that the DK government killed 100 percent of the urban and rural Vietnamese population. The Cham are a separate ethnic group in Cambodia and practice Islam. They were targeted in killings, but also forced to only speak Khmer, prohibited in practicing their religion, and their communities were dispersed, as were other ethnic minorities. This ethnic homogenization differed from the treatment of the Khmer, who were still allowed to speak their language, Khmer, and many Khmer villages were not dispersed whereas the above ethnic minorities were. These factors qualified as genocide.