suffix
A suffix attached to the name of a lake, e.g., 琵琶湖 (Lake Biwa). Not used as a standalone word for 'lake' (use 湖 みずうみ).
琵琶湖は日本で一番大きい湖です。
Lake Biwa is the largest lake in Japan.
諏訪湖のほとりを散歩した。
I took a walk along the shore of Lake Suwa.
The kanji 湖 means 'lake'. The reading こ is the on'yomi and is used in compound words and place names.