noun
carp; common carp; koi
Refers to the common carp (Cyprinus carpio), including ornamental koi varieties. In everyday Japanese, 鯉 is the general term for carp.
池に大きな鯉が泳いでいる。
A big carp is swimming in the pond.
鯉のぼりは端午の節句の飾りです。
Carp streamers are decorations for Children's Day.
Specifically refers to ornamental colored carp (koi), while 鯉 is the broader term for carp in general.
The kanji 鯉 is a phono-semantic compound: the fish radical 魚 combined with the phonetic component 里 (ri). The Japanese reading こい is native and its origin is uncertain.