The word "is" is a verb that serves as a form of the verb "to be." It is used to indicate existence, identity, or a state of being. For example, in the sentence "The sky is blue," the word "is" connects the subject, the sky, to its description, blue. In grammar, "is" functions as a linking verb, helping to connect the subject of a sentence with a subject complement. This complement can be an adjective, noun, or pronoun. For instance, in "She is a teacher," "is" links She to her identity as a teacher.