In reading, it's very common to come across words that are unfamiliar. Sometimes because an author is intentionally reaching for an unusual word, to give a sense of estrangement to the writing, and sometimes purely by chance. Certain kinds of writing are more likely to feature unfamiliar words: in my experience, scifi and fantasy are particularly heavy in odd words, probably for the sake of estrangement, and translated fiction is, too, probably due to the translator trying to capture some difference in sense in a foreign word.
So, here I will record some of the less-familiar words and phrases I come across, whether they're entirely new to me, or just stand out for their rarity or for being used in an unusual context.
As a little comparison, some stats on word frequencies (from COCA, via wordfrequency.info):
words per occurrence
What does this mean in real terms? A novel is on the order of 90k words on average, so you'd expect to read about three novels to encounter a particular word as rare as police at rank 10,015, and you'd expect to read about 275 novels to encounter a given word as rare as cavil at rank 61945.
Put another way, in each novel, you expect to see approximately:
one word from the bottom 500 on that list, ranks 61500–62000 (e.g. cavil)
two words from ranks 50000–50500 (e.g. thiamin)
three words from ranks 40000–40500 (e.g. ossification)
eight words from ranks 30000–30500 (e.g. thin)
22 words from ranks 20000–20500 (e.g. panacea)
81 words from ranks 10000–10500 (e.g. police)
and any given word from the top 2000 or so at least once
Besides these, you'd see a variety of proper nouns, nonce words, and others not included in the frequency list.
related to actinism, "That property of electromagnetic radiation that leads to the production of photochemical effects"
Occurs every 19 million words in American English, but once in 1.5 million in scifi and fantasy.
Relating to medicine. Aesculapius is the Greek god of medicine. It's an Aesculapian outburst because McCoy is a doctor, I guess.
A dated word for "an old woman, particularly an ugly one," though in this case I think Kris does not intend to call the women ugly.
In modern American English, beldame occurs once in about 110 million words, though in science fiction and fantasy it's much more common, appearing about once in seven million words.
No reason to use a fifty-cent word when a five-dollar word will do. Chalcedony is a stone which, in fact, comes in many colors. But Foster means a whitish variety, like this.
Occurs every 29 million words in American English, but once in 9 million in scifi and fantasy.