The expulsion of an electron from an atomic nucleus is called decay.

The expulsion of an electron from an atomic nucleus is called β⁻ decay.

β decay is a type of radiation and it consists of two parts as β⁻ decay and β⁺ decay.

β⁻  decay is a conversion of neutron into a proton by emission of an electron.
when an element undergoes a β⁻  decay, the daughter nucleus will have the same number of mass but the number of protons will be increased by 1 and number of neutron will be decreased by 1.

β⁺ decay is a conversion of proton into a neutron by emitting a positron. Then daughter nucleus will have same number of mass but proton number will be decreased by 1 and neutron number will be increased by 1.