Chemistry

A semiconductor is a solid whose electrical conductivity is in between that of a conductor and that of an insulator, and can be controlled over a wide range, either permanently or dynamically.

Semiconductor materials can be elements (e.g. germanium, silicon), compounds (e.g. copper oxide, lead sulfide), or metal alloys (e.g. gallium arsenide, gallium aluminum indium phosphide).

A pure semiconductor material is a very poor conductor or even an insulator, but when impurities called dopants are added it can become a quite good conductor. There are two classes of dopants: N type and P type. N type dopants create negatively charged current carriers (i.e. free electrons) in the semiconductor, just like ordinary conductors use. P type dopants create positively charged current carriers (i.e. holes) in the semiconductor. (Note: dopants do no make the semiconductor material electrically charged.) Because semiconductors have two different types of current carrier (not just the one that ordinary conductors have), it is possible to create junctions between differently doped parts of semiconductor material that conduct in one direction but not the other, or due to quantum mechanical effects do other useful things.
Semiconductor is a material that has the electrical conductivity between that of a conductor and an insulator. Silicon, germanium and graphite are some examples of semiconductors.

Source : Physics and Radio-Electronics ( website)
it is a solid material that has conductivity somewhere between a metal and that of a good insulator, due to addition of some impurity or due to temperature. Most electronic components are semiconductors.