No. Light can be further defined. One way is to say it is visible energy. And, thinking of that, I have had an enlightenment: the essence of light is visibility! Come on, I dare you all to come up with something better.

If essence is undefinable, asking after it is asking after nothing. There's something essentially appealing about nothing, not?

Or, otherwise: the essence of light is the definition of light provided the definition of light is undefinable.

The Hebrew "Ner" is "lamp": then the essence of light is that which makes it and we are back to Day One and Skokie.