Nonalpha characters are characters such as punctuation marks and symbols that appear on keyboards. Examples of nonalpha characters that are punctuation marks include the exclamation mark, a comma, a period and a colon. Examples of nonalpha characters that are symbols include the dollar sign, asterisk, percent sign and ampersand.
All the numbers and letters on a keyboard are referred to as alphanumeric characters. The other characters used in sentence construction that are neither numbers or the 26 letters of the alphabet are non-alphanumeric characters.
People use nonalpha characters when punctuating sentences and when writing certain symbols, such as the "@" symbol used in emails. Some nonalpha characters, such as the plus sign, the asterisk and the minus sign, are used in mathematical calculations or in Excel equations. These days, people also use nonalpha characters to write certain emoticons.
Another area where nonalpha characters are used is in password creation in various websites. Most websites consider a strong password to be one that has a combination of alphabets, numbers and nonalpha characters. Such passwords are harder for attackers to guess, and it is more difficult for them to discover such passwords using brute force, offline dictionary attack or online dictionary attack.