Definition of effective mass in US English:

effective mass

noun

  • The mass of something as calculated or inferred from its effect in a particular context; (Physics) the mass that needs to be assigned to a particle if the usual equations of motion are to hold in a situation where they do not strictly apply; specifically the mass that an electron appears to have in a crystal lattice, where its motion is affected by electric and magnetic fields.

Origin

Mid 19th century; earliest use found in Philosophical Transactions of the Royal Society of London.