What does hapax mean in Greek?

Thrice was I beaten with rods, once was I stoned, thrice I suffered shipwreck, a night and a day I have been in the deep;

For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost,

But into the second went the high priest alone once every year, not without blood, which he offered for himself, and for the errors of the people:

For then must he often have suffered since the foundation of the world: but now once in the end of the world hath he appeared to put away sin by the sacrifice of himself.

And as it is appointed unto men once to die, but after this the judgment:

So Christ was once offered to bear the sins of many; and unto them that look for him shall he appear the second time without sin unto salvation.

For then would they not have ceased to be offered? because that the worshippers once purged should have had no more conscience of sins.

Whose voice then shook the earth: but now he hath promised, saying, Yet once more I shake not the earth only, but also heaven.

And this word, Yet once more, signifieth the removing of those things that are shaken, as of things that are made, that those things which cannot be shaken may remain.

For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit: