A question mark is a punctuation mark or, more pedantically, a tone mark. It is a full stop, like a period used in interrogatory sentences, such as "Where did you get that hat?" In US English it is also called a query.
The symbol is generally thought to originate from the Latin quæstio, meaning question, which was abbreviated to 'Qo', which was transformed into the symbol.
In computer programming, the question mark corresponds to Unicode and ASCII character 63, or 0x003F.
For the rock music band, see ? & the Mysterians.