f ∝ 1/r
Zipf's Law states that in any natural language corpus, the frequency of a word is inversely proportional to its rank. The most common word appears roughly twice as often as the second most common, three times as often as the third, and so on — a power law that holds across languages, texts, and even music.
Hapax legomena (from Greek: "said only once") are words occurring exactly once in a corpus. In most natural texts, over 40–60% of unique words are hapax. High hapax ratios suggest rich vocabulary or sparse data.