A semiconductor is a material (most often an element like silicon, but sometimes a compound like galena, used in early diodes) which is intermediate in its ability to carry current between an insulator (e.g. most plastics) and a conductor (e.g. most metals).
Semiconductors are very important to modern electronics because the addition of trace amounts of dopants (other elements) can render them into materials which have an excess of electrons (so-called 'N'-type semiconducors) or holes (the absence of an electron, so-called 'P'-type semiconducors).
Joining together blocks of P- and N-type semiconductors in the appropriate configurations can produce a wide range of devices, including diodes and transistors. The ability to create multiple such devices on a single small block of semiconductor material allowed the creation of integrated circuits.
The term may also be used to refer to sub-systems constructed using semiconductor devices; e.g. at one point all main memory was core memory, so the term 'semiconductor RAM' was used to indicate the use of ICs to actually hold data.