What is the mass of an electron?
As the electron is quite small, its true mass changes as it moves, and we call that the relativistic mass.
#m_"rel" = m_"rest"/sqrt(1 - v^2/c^2)#
However, as the time-independent Schrodinger equation involves what are called stationary states, we tend to use its rest mass in calculations, defined as
#m_"rest" -= m_e = 9.10938291_(40)xx10^(-31) "kg"#,
where the subscript indicates the digits past the ones to which we are certain.