noun which may take the genitive case particle 'no', noun
ancestral; hereditary; handed down from generation to generation
Used as a noun or no-adjective to describe something that has been passed down within a family for many generations, such as property, traditions, or heirlooms.
この土地は先祖代々のものです。
This land has been handed down in our family for generations.
先祖代々の言い伝えを守っている。
We keep the ancestral tradition that has been passed down for generations.
adverb
for generations; from generation to generation
Adverbial use describing an action or state that continues over many generations within a family.
この家は先祖代々続いてきた。
This family has continued for generations.
先祖代々受け継がれてきた技を教える。
I teach the skill that has been passed down from generation to generation.
代々 means 'for generations' or 'successive generations' and can be used more broadly, not necessarily limited to family ancestors. 先祖代々 specifically emphasizes ancestral lineage.
世襲 refers to hereditary succession of a position or title, often in a formal or institutional context, while 先祖代々 is more about family inheritance in general.
Compound of 先祖 (ancestor) and 代々 (for generations). The iteration mark 々 repeats the preceding kanji 代, so 代々 is equivalent to 代代.