A nonalphanumeric character is any symbol on a keyboard that is neither a number nor a letter, such as punctuation and mathematical symbols. For increased security, some password conventions require users to include at least one non-alphanumeric character in their passwords.
Nonalphanumeric characters include punctuation marks such as commas, periods, exclamation points, quote marks, colons, question marks, and semicolons, which are sometimes referred to as nonalphanumeric characters. Other frequent nonalphanumeric keyboard characters include dollar signs, ampersands, asterisks, plus signs, equals signs, underscores, tildes, percentage symbols, and slashes. A nonalphanumeric character is any sign on a keyboard that is not part of the 26-letter alphabet or a numeric character (usually 0-9).