The Thunder Materia is an enemy in Crisis Core -Final Fantasy VII-. Sephiroth summons it, along with three others, to defend himself. The elemental Materia cast spells based on their element up to -ga level, and resist the element they are composed of as well as physical attacks.
Thunder is the sound caused by lightning. Depending on the distance and nature of the lightning, thunder can range from a sharp, loud crack to a long, low rumble (brontide).
Materia means "matter" or "substance" in Latin, Spanish and Italian; it also means "subject" in Italian and Spanish, and is the ascendant of Portuguese "Matéria".