The term ONE BILLIONTH (PREF.) can be solved by the crossword solution NANO, which is a four-letter word. The prefix 'nano' is derived from the Greek word for 'DWARF' and is used to denote a very small quantity or size. In scientific terms, it is used to refer to one billionth of a unit, such as one billionth of a meter. Therefore, the crossword solution NANO is an appropriate answer for the term ONE BILLIONTH (PREF.).