A negator is a morph that expresses negation and does not simultaneously express a root meaning (event, thing, property).
A largely equivalent term is "negative marker", but "negator" is preferred here, because a negator is not a grammatical marker in the Grammaticon sense (it does not have grammatical meaning as defined here).
Moreover, grammatical markers are by definition bound forms, whereas negators may occur freely as response words ('no!').