A somewhat literary or formal term for thunder, often used in written or poetic contexts. In everyday speech, 雷 (かみなり) is more common.
Thunder roared and heavy rain began to fall.
Compound of 天 (てん, 'heaven') and 雷 (らい, 'thunder'), literally 'heavenly thunder'. The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.