port·man·teau word
noun
plural noun: portmanteau words
a word blending the sounds and combining the meanings of two others, for example motel (from ‘motor’ and hotel’) or brunch (from ‘breakfast’ and ‘lunch’).
A portmanteau word is a combining of two words into one word with a similarly collective meaning. In this case 'null hypothesis" and 'theist' have been blended into null hypotheist.