Question

Tell me some phsical quantities which we hear rarely with definition and units.

Answer 1

1) the light-year which are quite "usual"
2) One rack unit (U) is 1.75 inches (44.45 mm) and is used to measure rack-mountable audiovisual and computing equipment.
3)One barn is 10−28 square metres, about the cross-sectional area of a uranium nucleus.
etc