Gairaigo

Gairaigo (kanji: 外来語, hiragana: がいらいご) are loanwords in the Japanese language from languages other than Old or Middle Chinese. The word itself literally means "loan word". Most "gairaigo" come from European languages, and especially English since the post-WWII era, though Modern Chinese and Modern Korean are also popular sources for loanwords. Most gairaigo nowadays are written in katakana, with some older ones being written in ateji (kanji used only to show a word's pronunciation and not its meaning) or even in hiragana. For example, while America, or "Amerika" in Japanese, can be written as 亜米利加 in ateji, it is rare to do so these days, and instead it is almost always written in katakana as アメリカ.
"Gairaigo" is one of the three main sources of Japanese words, along with "kango" (kanji: 漢語, hiragana: かんご), or Chinese loanwords, and "yamato kotoba" (kanji: 大和言葉, hiragana: やまとことば), or native Japanese words.
"Gairaigo" history.
Japanese has a long history of borrowing words from other languages. Since the Chinese character system gave Japanese a writing system (as Japanese had no known systems of writing beforehand), the Japanese naturally borrowed many Chinese words into their language. However, the Japanese only borrowed words from other Asian languages until the Portuguese and the Dutch came to Japan during the 16th century. Since the Portuguese and the Dutch traded many different goods with the Japanese at this time, the Japanese naturally included many Portuguese and Dutch words into their own language, including "karuta" (ateji: 加留多 or 歌留多, hiragana: かるた, katakana: カルタ) from the Portuguese "carta" meaning "letter", "tempura" (ateji: 天麩羅 or 天婦羅, ateji and hiragana [most common form]: 天ぷら, hiragana: てんぷら, katakana: テンプラ), from the Portuguese "tempora" meaning "temperance", as Catholics would not eat red meat during Lent, and "tabako" (kanji: 煙草, hiragana: たばこ, katakana: タバコ) from the Portuguese "tabaco" meaning "tobacco". While these terms used to be written mostly in ateji (and they still can be), they are now mostly written only in hiragana or katakana.
During the Meiji Period, after Matthew Perry forced Japan to open trade with other countries, Japan sent many scholars to Western countries to study different field of knowledge, such as Western medicine in Germany, an art in France. However, since the end of the 19th century, and especially after WWII, most "gairaigo" came from English. Today, about 90% of all "gairaigo" come from English.
Japanese-invented English words.
While many "gairaigo" came from English, Japanese has also invented many new words based off of English, but wouldn't be understood by native English speakers. These are called "wasei eigo", or "Japanese-made English words". Many of them are shortenings of English words such as "amefuto" (katakana:アメフト) which is a shortening of "American football", "pasokon" (katakana:パソコン) which is a shortening of "personal computer", "konbini" (katakana:コンビニ) which is a shortening of "convenience store", and "anime" (katakana: アニメ) which is a shortening of "animation". While each of these words have a longer gairaigo word that go along with them which would be "amerikanfutobōru" (katakana:アメリカンフットボール), "pāsonarukonpyūtā" (katakana:パーソナルコンピューター), "konbiniensusutoa" (katakana:コンビニエンスストア), and "animēshon" (katakana:アニメーション) in that order, they would be too long for a Japanese person to say in conversation. Also, some words got a new meaning when they come into Japanese. For example, "consent", or "konsento" (katakana:コンセント) in Japanese, means "electrical outlet" rather than its original meaning, and "range", or (katakana:レンジ) in Japanese, means "stove" rather than its original meaning.