If I had to summarize what a Druid is in one sentence, the sentence might be:

"A Druid is a professional invigilator of living spiritual mysteries as expressed by Celtic cultural forms".

This is the most precise and yet fully encompassing definition that I can concieve of at this time. It doesn't employ vague metaphor, but uses analysable language, and it doesn't harken back to a claim about history which means that this definition may serve contemporary people. But even so, perhaps it deserves further elabouration.

"professional"
because being a Druid is a responsibility for one's tribe, not only for one's own self. Being a professional means having a skilled capacity that not everyone has yet everyone needs; it is the investment in you by one's tribe so that you could aquire that skilled capacity, that grounds you in responsibility to that tribe. Being a Druid requires one to be accessible and available to people whose scope of spiritual vision is not as wide as yours. Among one's responsibilities to such people is to aid them to widen their scope of vision. Being a Druid is even a responsibility to the world itself, for as the ancient Druids said, "we created the world", and without Druids to bring about the renewal of the seasons with their rituals, the world might end. So goes the myths. But even contemporary Druids have responsibilities to the world for our Earth is dying. She is being poisoned by the excrement of human industry.

"invigilator"
a word encompassing a range of ideas, including steward, investigator, watcher, even 'knower', but also operator and user. An invigilator is a person who keeps a vigil, which means watchful and mindful and attentive over something. And so to say that a Druid is an invigilator is to say that a Druid is watchful and mindful of something.

"living"
to emphasise that the spiritual mysteries are real and accessible, and not locked in an unknowable, unreachable heavan but manifest and realised in our embodied world. (but this is connected to my idea that the Celtic "otherworld" is not really a mythic place but actually a certain state of consciousness.)

"spiritual mysteries"
Universal principles of animation that emerge from all environmental life. (see my essay, "Ex Nemeton 1: On Mysteries and Spirit") Spiritual mysteries exist everywhere and always, and are knowable by everyone. They are the life-experiences and world-forces that give shape to reality and meaning to our existance. Yet there needs to be something more saying that there is a particularly Druidic way of knowing them. Hence the last part of the definition, which reads:

"as expressed by Celtic cultural forms"
being the culturally specific content of my definition, so to include poetry, art, archaeology, architecture, literature, mythology, language, and folklore, and so to require Druids to have comprehensive knowledge of most of these features of Celtic culture. This is also a strictly epistemic issue, for it touches on ways of knowing about things, which is different from the issue of what things essentially *are* (that's metaphysics).

Postscript: The reporter gently suggested that the definition I gave him was not simple enough for the general public, and so I produced this:

Druidism is the revival of the ancient Celtic religion which holds the Earth and the environment as sacred, and promotes a morality based on truth, honour, strength, and justice.

I believe this is simple enough to be understood by all, and is fairly representative of both ancient Druidism and modern revival Druidism. But it too deserves further elabouration-- what is meant by Truth here, for example. But this is work for another day.