There are 2 types of physical quantities, the and ones.
Fundamental quantities are those which do not depend upon other physical quantities, there are . They have own defined units.
are some function of the , and they are infinite in number. Their units are depending on the units of in their dimensional analysis.
That's how quantities are defined, we all know this.
But do you know how are defined ?
(Note, here I don't mean that is defined as )
The mass of a platinum-iridium cylinder kept in the National Bureau of Weights and Measurements, Paris
The distance travelled by light in vacuum in part of a second or also defined as 1650763.73 times wavelength emitting from
The time interval in which Cesium-133 atom vibrates times.
fraction of thermodynamic temperature of triple point of water.
The amount of electric current that produces a force of per unit length, that acts between two parallel wires of infinite length and negligible cross-section area placed at 1m distance in vacuum.
The amount of intensity on area of blackbody in the direction perpendicular to its surface at freezing point of platinum K at pressure of
The amount of substance which has same number of elementary entities as in 12 gm of carbon-12.
Source- NCERT textbook. (Felt like sharing because this was new to me and may be new to many of us!)