Length Units Si -meter, gs - centimeter, cm US Customary -foot, ft a Defined in terms of a meter - the distance traveled by light in a vacuum during a given timeLength ◼ Units ◼ SI – meter, m ◼ cgs – centimeter, cm ◼ US Customary – foot, ft ◼ Defined in terms of a meter – the distance traveled by light in a vacuum during a given time