Swimming is a healthy exercise. (Identify the word class of the underlined word 'Swimming')

A gerund is a verb form ending in -ing that functions as a noun. In this sentence, 'Swimming' is the subject of the verb 'is' and refers to the activity of swimming as a thing/concept. Since it is derived from a verb but functions as a noun (the subject), it is a gerund. It is not acting as a verb here, nor as an adjective or adverb.