noun
plunge basin; waterfall pool
The deep pool formed at the base of a waterfall by the force of falling water.
滝壺の水はとても冷たかった。
The water in the plunge basin was very cold.
彼は滝壺に飛び込んだ。
He jumped into the waterfall pool.
滝 means 'waterfall' in general, while 滝壺 specifically refers to the basin at its base.
Compound of 滝 (taki, 'waterfall') and 壺 (tsubo, 'pot, basin'). The reading つぼ becomes voiced to づぼ (dzu > zu) due to rendaku.