Hapax legomenon
Hapax legomena are quite common, as predicted by Zipf's law, which states that the frequency of any word in a work (corpus) is inversely related to its rank in the frequency table.  For large corpora, about 40% to 60% of the words (counting by type) are hapax legomena, and another 10% to 15% are dis legomena. In the Brown Corpus of American English, about half of the 50,000 words are hapax legomena within that corpus.