「暑い」 (Atsui) is an i-adjective specifically meaning 'hot' when referring to the weather or climate. You use it to describe the temperature of the air, indicating warmth or high heat outdoors. For example, a hot summer day.

「暑(あつ)い」は「hot」という意味(いみ)で、特(とく)に「天気(てんき)」が熱(あつ)いときに使(つか)われるい形容詞(けいようし)です。