"Inlet" (visual definition)

Cf. verbal definitions e.g. "a passage, valve, or part through which a substance, esp a fluid, enters a device or machine".
https://www.thefreedictionary.com/inlet

(From Eggedal Mølle, a museum in Eggedal, Southern Norway. "Previously, it had been a small water mill and is the only water-powered mill left in Norway. The mill was built in 1912 and was in operation until 1965. The mill was restored and put into operation again with hydropower." https://en.m.wikipedia.org/wiki/Eggedal)