Coefficients used in the quantum theory of radiation,
related to the possibility of a transition occurring
between the ground state and an excited state in the
processes of induced emission and spontaneous emission.

For an atom exposed to electromagnetic radiation,
the rate of absorption R_{a} is give by R_{a}
= B_{p}, where p is the density of electromagnetic
radiation and B is Einstein B coefficient associated
with absorption

The rate of induced emission is also given by B_{p}
with the coefficient B of induced emission being equal
to the coefficient of absorption.

These coefficients were put forward by Albert Einstein in 1916 -17 in his
analysis of the quantum theory of radiation.