Thunder is an Alignment.
It is strong against the Water Alignment and weak against the Earth Alignment.
It appears in the games Yu-Gi-Oh! Forbidden Memories, Yu-Gi-Oh! Dark Duel Stories, Yu-Gi-Oh! The Sacred Cards, and Yu-Gi-Oh! Reshef of Destruction. It is also briefly featured in the manga.
In the OCG and TCG, Thunder is the Type version of this Alignment.