In modern terminology the mass of an object is its invariant mass, which is zero for a photon.
What is relative mass of photon?
ii) The photon has REST mass equal to 0. Its relativistic mass is related to its energy by m=E/c^2 and is non zero.
What is resting mass?
Rest mass (physics), the mass of a body as measured when the body is at rest, but it is relative to an observer who is moving or not, an inherent property of the body. All matter, such as any object, has some rest mass.