Penultimate means "occurring immediately before the last one," or in other words, "next to last." A formal adjective, it is always used before the noun it modifies.

// The penultimate episode of the TV series features some shocking plot twists that set up what will surely be a thrilling series finale.

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"The high school soccer playoffs are down to their penultimate round with the state semifinals looming later this week after a select few regional champions were crowned on Tuesday." — Matt Welch, The Plano (Texas) Star Courier, 2 Apr. 2025

Penultimate isn't the last word in words for things that are next to last. It has a pair of closely-related noun synonyms: penult and penultima. Although all three concern something that's next to last, penult and penultima are usually a bit more specific; they are used most often to identify the next to last syllable of a word. All three come from paenultima, the feminine of paenultimus, a Latin root from paene ("almost") and ultimus ("last"). You may occasionally hear the word penultimate used as an intensified version of ultimate, as in "a race they've called 'the penultimate challenge.'" This use isn't typically found in edited prose, however, or in dictionaries, as discussed in this video.