noun
alphabet
Refers to a set of letters used in a writing system, especially the Latin alphabet. Common in everyday contexts.
英語のアルファベットは26文字です。
The English alphabet has 26 letters.
アルファベット順に並べてください。
Please arrange them in alphabetical order.
文字 is a broader term for any character or letter, including kanji and kana, while アルファベット specifically refers to an alphabet set.
Loanword from English 'alphabet', ultimately from Greek 'alpha' and 'beta'.