(i) Meter: The presently accepted definition of meter is the length of path travelled by light in space in 1/299,792,458th second.
(ii) Kilogram: Kilogram is the fundamental unit of volume. It is explained as the mass of a particular cylinder of platinum - iridium kept at the International Bureau of Weights and Measures in Paris.
(iii) Second: Second is the fundamental unit of time. It is explained as 86,400th part of seem solar day. Second is accurately measured by an atomic clock.