The crossword solutions for the term 'Fleeting Prefix, Timewise' are 'Nano' (4 letters). The prefix 'nano-' is derived from the Greek word for 'DWARF' and is used to denote a very small quantity or size. It is often used in scientific contexts to refer to something that is extremely small or short-lived. In this case, it is being used to describe something that is fleeting or short-lived in terms of time.