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