unique, such that it cannot be mistaken for something else.
1895, H. G. Wells, The Time Machine Chapter XNow, I still think that for this box of matches to have escaped the wear of time for immemorial years was a strange, and for me, a most fortunate thing. Yet oddly enough I found here a far more unlikely substance, and that was camphor. I found it in a sealed jar, that, by chance, I supposed had been really hermetically sealed. I fancied at first the stuff was paraffin wax, and smashed the jar accordingly. But the odor of camphor was unmistakable.
