(n.) an overwhelming feeling of awe or astonished wonder
A feeling of mind-boggling awe or wonder is tremendum.
Unsurprisingly, that’s a Latin word; in fact, it’s a Latin word hijacked from a longer Latin phrase, mysterium tremendum, that essentially means a ‘terrible mystery’.
Terrible here doesn’t quite carry the same sense we tend to use it today, admittedly. A mysterium tremendum is really a profound mystery, or a mystery so wondrously impressive or awe-inspiring that it feels beyond our comprehension. In that context, this expression was much used among nineteenth-century theologians and theological students to describe the mind-boggling wonder experienced when contemplating the abilities, powers and miracles of God.
From there, mysterium tremendum was clipped just to tremendum in the early 1900s, and has been in use (albeit rarely) in a slightly weaker, more generalized sense ever since.
Etymologically, tremendum is of course related to tremendous—but it’s also related to words like tremor and tremulous. At their root lies a Latin word, tremere, meaning to quake or shake, typically with some kind of overwhelming emotion. Originally many of the English words descended from this root were in some way negative—something tremendous would once have terrified you, not impressed you, for example. But as their meanings advanced over time, they steadily became positive.
(Oddly, awe itself went the other way in the word awful, which originally described something literally so full of awe and wonder that it commanded respect. Overtones of something being so ‘awe-full’ that it caused dread sent that particular word on a more negative trajectory.)