univerbation

noun

uni·​ver·​ba·​tion ˌyü-ni-vər-ˈbā-shən 

: the process by which a fixed collocation of words becomes a single word (such as the verb atone from the phrase at one, or the adjective dyed-in-the-wool from the phrase dyed in the wool)

Word History

Etymology

probably borrowed from French, from uni- uni- + Latin verbum "word" + French -ation -ation — more at word entry 1

First Known Use

1965, in the meaning defined above