What part of speech is the word 'slowly'?

The word “slowly” is an adverb.

In a sentence such as “The turtle moved slowly,” it modifies the verb “moved” by describing the manner of the action. Adverbs can also modify adjectives, other adverbs, clauses, or whole sentences, expressing ideas such as time, place, frequency, degree, and certainty.

Many English adverbs of manner are formed by adding -ly to an adjective: “slow” becomes “slowly,” “quiet” becomes “quietly,” and “careful” becomes “carefully.” The pattern is useful but not universal. Words such as “fast,” “hard,” “well,” “here,” and “yesterday” also serve as adverbs, while some -ly words, including “friendly” and “lovely,” are adjectives.

“Slowly” should not be confused with a preposition, which introduces a relationship such as “under the table,” or with “slow” used as a flat adverb in expressions such as “drive slow.” Traditional grammar calls adverbs a part of speech, though modern linguists note that the category contains several different kinds of words.