noun
symbolic character; special character
Computing term for characters that represent symbols, such as punctuation, mathematical operators, or other non-alphanumeric glyphs.
パスワードには記号文字を含めてください。
Please include a symbolic character in your password.
このフォントは記号文字が豊富だ。
This font has a rich set of symbolic characters.
特殊文字 is a broader term for any special character, while 記号文字 specifically refers to characters that are symbols rather than letters or digits.
Compound of 記号 (symbol) and 文字 (character).