The base or main part that something is built on.
The house's fundament is its foundation.
その家の基盤はその基礎です。
The main idea or principle that something is based on.
The fundament of her argument was solid evidence.
彼女の主張の基盤は確かな証拠でした。
The buttocks.
He fell on his fundament.
彼はお尻で転んだ。