The nanometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American spelling) is a unit of length in the metric system, equal to one billionth of a metre. The name combines the SI prefix nano- (from the Ancient Greek , , “dwarf”) with the parent unit name metre (from Greek , , “unit of measurement”). It can be written in scientific notation as , in engineering notation as , and is simply . One nanometre equals ten ångströms.