Typically momentum is thought as mass times velocity, and since photons do have momentum, there was a desire to give them some kind of "relativistic mass". In

Typically momentum is thought as mass times velocity, and since photons do have momentum, there was a desire to give them some kind of "relativistic mass". In more recent times, it has been seen as easier to use just one concept of mass, and to redefine momentum entirely. So, photons have a mass of 0, and we don't need to specify "rest mass". But they do have momentum.