noun
ancestral tablet; memorial tablet for ancestors
A Buddhist memorial tablet inscribed with the posthumous name of an ancestor, kept on the family altar (仏壇) and venerated.
お盆には先祖の位牌に手を合わせる。
During Obon, we put our hands together in prayer before the ancestral tablets.
Compound of 先祖 (ancestor) and 位牌 (memorial tablet). The term is straightforward and its meaning is transparent from its components.