This is a common ideogram in modern Western systems. As
a sign used in trade and commerce it means number. It is also common on computer
printed forms. Its name in English is octothorpe, a name associated with hamlet.
In musical notation it is called a sharp and indicates that one should raise a half tone. Another, similar sign, , means cancel the raised half tone or cancel the lowered tone indicated by .
The sign has also been used in early chemistry to denote air.
In medicine the sign is sometimes used to indicate a fracture, i.e. a broken bone.
For the meaning and use of on telephone keyboards, see in Group 45.