superlative is the degree of grammatical comparison that denotes an extreme or unsurpassed level or extent — the form of an adjective or adverb expressing the highest degree, as in nicest, worst, or most interesting.
English grammar recognizes three degrees of comparison. The positive is the base form of the adjective or adverb (nice), the comparative ranks one thing against another (nicer), and the superlative marks the extreme of the set (nicest). Short words typically take the suffix -est; longer words take most or least instead of a suffix, which is why the superlative of interesting is most interesting rather than a single word. A handful of common adjectives are irregular and supply their own forms — good becomes best, bad becomes worst, little becomes least.
Beyond grammar, the word carries two everyday senses. As an ordinary adjective, superlative means of the highest quality or degree, surpassing all others. As a noun, a superlative is an extravagant expression of praise — the sense at work in the phrase speaking in superlatives, which describes someone whose every description reaches for the extreme. That usage sits close to 📝hyperbole: a superlative asserts the top of a real scale, while hyperbole abandons the scale altogether, and habitual reaching for the top of the scale tends to collapse into the second.