In grammar, a word used to describe a characteristic of a person, place or thing is known as an adjective. Therefore, a one-word description of a person is an adjective that identifies a particular quality or attribute about that person.
"Nervous," "thoughtful" and "respectful" are examples of one-word adjectives that describe people. In the sentence, "John was nervous before the big game," for instance, the word nervous is an adjective used to describe how John felt right before the big game. In contrast, someone could describe John as being "anxious," "excited" or "elated" before the big game. Adjectives include positive and negative characteristics.