The square centimetre is a unit of area and is a multiple of the square metre; an SI derived unit. It has the symbol cm².
A square centimetre is equal to the area of a square with edges 1cm long. There are 10,000 square centimetres in a square metre.
It is often used to describe areas smaller than 1m² - including much work in classrooms on paper.
The barn is a unit of area and is used in particle physics with the symbol b.
It represents approximately the area of the cross-section of a uraneum nucleus, so is very small compared to most other recognised units of area.
It is also used to measure the interaction of a nucleus with an electric field gradient in nuclear quadrupole and magnetic resonance.