A covalently bonded compound, in which the atoms form a single covalently bonded structure (like a giant Molecule), often a Crystal.

http://www.bbc.co.uk/education/guides/z94xsbk/revision/4

Some properties

• Very high melting points – this is because a lot of strong covalent bonds must be broken. Graphite, for example, has a melting point of more than 3,600°C.
• Variable electrical conductivity - diamond does not conduct electricity, whereas graphite contains free electrons so it does conduct electricity. Silicon is a semi-conductor – it is midway between non-conductive and conductive.