I’m ashamed to admit that I usually just keep on reading. LOL If I can’t figure it out by the context in which it’s used I’ll move on. Although, I do feel the need to mention that it’s rare for me to get stumped on a word so I don’t want you thinking that I’m constantly skipping over important parts of books.
I read paranormal fiction not scientific textbooks so generally the vocabulary isn’t overly above-average. The only time that I’ll actually look up a word is if I’ve come across it multiple times and still don’t know the meaning or if it’s world-specific vocabulary and the novel includes a glossary of terms (like in J.R. Ward’s BDB series).