If something’s the penultimate, then it’s the second to last thing in a series. If you’re watching the penultimate episode of your favorite TV show, you know that there’s only one more episode to go.
Penultimate came into English in the 17th century from the Latin word paenultimus, a combination of paene, meaning “almost,” and ultimus, meaning “last.” So if something’s penultimate, it’s “almost last”: more specifically, it’s next to last. If you’re reading a book with 20 chapters, chapter 19 is the penultimate chapter. If you’re a fan of grammar terms, you might already know that penultimate can refer to a word’s next to last syllable, called a penult.