What is what part of speech is is?

The word "is" is a verb. Specifically, it's a form of the verb "to be".

• It acts as a linking verb (also sometimes called a copula) when it connects the subject of a sentence to a noun or adjective that describes or renames the subject. For example, in the sentence "The sky is blue," "is" links the subject "sky" to the adjective "blue". To learn more: Linking%20Verb

• It can also be an auxiliary verb (helping verb) used to form different tenses and voices of other verbs. For example, in the sentence "She is running," "is" helps form the present continuous tense. To learn more: Auxiliary%20Verb

• "Is" is the third-person singular present tense form of the verb "to be." The other forms include: am, are, was, were, be, being, been. To learn more: Verb%20Conjugation