Words containing "ai"
aidokusho 愛読書 --- one's favorite book
aijin 愛人 --- lover
aijou 愛情 --- love, affection
aikenka 愛犬家 --- a dog lover
aikokushin 愛国心 --- patriotism
aisha 愛車 --- one's cherished car
aiyousuru 愛用する --- to use habitually
boseiai 母性愛 --- mother's love, maternal affection
hakuai 博愛 --- philanthropy
Words containing "koi"
hatsukoi 初恋 --- first love
hiren 悲恋 --- sad love
koibito 恋人 --- one's boyfriend/girlfriend
koibumi 恋文 --- love letter
koigataki 恋敵 --- a rival in love
koi ni ochiru 恋に落ちる --- to fall in love with
koisuru 恋する --- to be in love with
renai 恋愛 --- love
shitsuren 失恋 --- disappointed love
"Koi" is a love for the opposite sex, or a feeling of longing for a specific person. It can be described as "romantic love" or "passionate love." While "ai" has the same meaning as "koi," it also has a definition of a general feeling of love. "Koi" can be selfish, but "ai" is a real love. Here are some lines that explain them well: Koi is always wanting. Ai is always giving.
"Renai （恋愛）" is written with the kanji characters of both "koi" and "ai." This word means, "romantic love." "Renai-kekkon （恋愛結婚）" is a "love marriage," which is the opposite of "miai-kekkon (見合い結婚, arranged marriage)." "Renai-shousetsu （恋愛小説）" is "a love story" or "a romance novel." The title of the movie, "As Good As It Gets" was translated as "Renai-shousetuska (恋愛小説家, A Romance Novel Writer)."
Here are some proverbs that include "koi."
"Soushi-souai （相思相愛）" is one of the yoji-jukugo （四字熟語）. It means, "to be in love with one another."
The Japanese sometimes use the English word "love" as well, though it is pronounced as "rabu （ラブ）" (since there is no "L" or "V" sound in Japanese). "A love letter" is usually called "rabu retaa （ラブレター）." "Koibumi （恋文）" sounds a bit dated, and it is more of a literary word than a conversational word. "Rabu shiin （ラブシーン）" is "a love scene". Young people say "rabu rabu (ラブラブ, love love)" when they are very much in love.
"Ai （愛）" can be used as a female name. Japan's new royal baby was named Princess Aiko, which is written with the kanji characters for "love （愛）" and "child （子）." Click here to read more about Princess Aiko. However, "koi （恋）" is rarely used as a name.
There are another words pronounced the same as "ai" and "koi". Since their meanings are distinctly different, I don't think there is any confusion between them when used in the proper context. With different kanji characters, "ai （藍）" means, "indigo blue," and "koi （鯉）" means, "carp." Carp streamers that are decorated on Children's Day (May 5th) are called "koi-nobori （鯉のぼり）."