source (of a spring, etc.)
source (of payment, energy, knowledge, etc.); origin; wellspring
温泉の源泉を見に行った。
I went to see the hot spring source.
彼女の創造力の源泉は読書だ。
Reading is the source of her creativity.
長年の友情が彼の活力の源泉になっている。
Long years of friendship have become the source of his vitality.