This is the 2049th most frequent Japanese word.
人工
Artificial; man-made.
Here, "人工" describes the lake as man-made, emphasizing that it is not naturally occurring.
この湖は人工の湖です。
This lake is an artificial lake.
In this sentence, "人工" is part of the compound "人工知能" (artificial intelligence), referring to man-made or synthetic intelligence as opposed to natural intelligence.
私は人工知能を研究しています。
I am conducting research on artificial intelligence.
"人工" is used in "人工的" to indicate something that is created or manipulated by humans, contrasting with natural methods or occurrences.