ancestor (senzo 先祖, so 祖)
1. Biological forebears; deceased family members from whom one is descended (in East Asia, patrilineally).
In Japanese Zen texts, senzo 先祖 always has this meaning, and generally refers to ancestors of lay parishioners of a temple.
2. Spiritual forebears; ancestral teachers (soshi 祖師) in the Zen lineage, from whom living members of the lineage have inherited the dharma. In Japanese Zen texts, the character so祖 when it stands alone usually has this meaning.