(Or "pictograph") A symbol which is a picture that represents an object or concept, e.g. a picture of an envelope used to represent an e-mail message.
Pictograms are common in everyday life, e.g. signs in public places or roads, whereas the term "icon" is specific to interfaces on computers or other electronic devices.
Pictograms are the most common kind of ideogram (symbols representing concepts), the other kind are not pictures but are conventions.