Another definition one could make is that light is 'non existent' unless interacting with something else. Existent but not measurably so, until its interaction. A 'probability field'
And even then you won't really be able to decide what it means. As you are free to define all momentum and freed energy to yourself as well as only to the one colliding with you, or anything in between.
In relativity your relative speeds, you being at rest with something else, doesn't matter for that rest mass you define. It's the same all the way to a collision, in where the energy freed will be a result of momentum's interacting.
The most common disagreement with being at rest with a light quanta is that it never can be said to be so. But that is just as true about anything of mass. the only rest we can define is relative something else, an 'absolute rest frame' can't exist unless you have a way of defining what it means universally.
It's even worse that this, depending on your faction. Light as waves, or as particles, or both. From a wave perspective it red shifts into oblivion, I have still to see that explained through a quanta perspective. so I go for the third definition, lights duality.
One thing we define though is that at relative rest with something you can find a restmass, and energy, that is its own, undisturbed by other circumstances. From that follows both red and blue shifts as becoming examples of 'things' interacting with it. As well as if you were able to be at rest with a light quanta you should find it to be the same, no matter what anyone else would like to define it as. So do a accelerating expansion redshift light into oblivion?