What part of speech is superlative?

"Superlative" itself is not a part of speech. It is an adjective that describes the highest degree of something.

However, superlatives are a form of adjective used to compare three or more things and indicate the highest degree of the quality being compared.

For example:

* tall (adjective) - taller (comparative adjective) - tallest (superlative adjective)

Here, "tallest" is the superlative form of the adjective "tall."