Radiation is a concept usually defined as moving outward in all directions from a source object. In physics, the word is used and defined differently in different contexts. For example a stone of Uranium-238 decays and radiates alpha particles, that is helium nuclei. Helium atoms successively assume two or more locations further and further away from the stone in all directions.
Radiation In context of light think is a little more tricky. Think of radiation as that which an atom does. An atom radiates. How an atom does this is as of now not clearly explained, though it is rigorously described using mathematics, for example the inverse square law of light, and perhaps quantum mechanics.
When we try to explain radiation one obvious question is how or why does a simple atom, say an H atom radiate in all possible directions??? And what is it that mediates this radiation??? The answer seems to be given in an assumption, namely the assumption that all the H atoms of the Universe are interconnected by a fundamental subatomic object that mediates light and gravity, and constitutes them all. This fundamental object has singular properties, and this assumption helps to answer the question of why an atom radiates in all possible directions in an inverse square regime. The atom taps into and works these fundamental subatomic objects in the act of radiation.
Ask yourself, where does light begin and where does it end???. . . . The answer is that is always begins and ends at the atom. If say in one direction there was a deprivation of Hydrogen atoms, then an atom wouldn't be able to "radiate". It seems that at least from a point of view in our location all the atoms of the Universe are spread out more or less evenly in all directions in a sort of imaginary sphere. Because of this an atom is able to radiate because it has a fundamental subatomic object connected to all possible atoms of the Universe. But if we travel far out to the last galaxy and last atom in any given direction, perhaps those atoms could not 'radiate' as the atoms at our location do. As Gaede brilliantly speculated in his book, Why God Doesn't Exist, the atoms at the edge of the Universe would be misshapen. And they would only consummate emission events in a one direction, not all directions. And perhaps these atoms emit in exotic atomic electron transitions.