also: もんじ
noun
Refers to individual symbols of a writing system, such as letters of an alphabet, kanji, kana, or other characters.
この文字は何と読みますか。
How do you read this character?
アルファベットは26文字です。
The alphabet has 26 letters.
noun
writing; written text
Refers to writing as a medium or to written content, often contrasted with speech or images.
文字で説明するのは難しい。
It's difficult to explain in writing.
彼は文字が読めない。
He can't read written text.
From Middle Chinese 文字 (mjun dziH), meaning 'written character'. The reading もじ is the regular Japanese adaptation, while もんじ preserves an older Chinese-influenced pronunciation.