But I suspect that only a few have heard the term before.

Don't worry, they're still around. It's basically a coil of wire, wrapped around a piece of iron (or some other magnetic material) with an current going through the wire. When the current is in one direction, there is a magnetic field in the centre of the coil. If you have a piece of magnetic material in the centre, it will want to move in the direction of the magnetic field. If you reverse the direction of the current, the piece of metal in the middle will move the other way. So if you use a solenoid as a switch, say to start something, when you apply the voltage, the metal in the middle shoots in one direction (as in a car starter). If you use it with alternating current (and thus alternating the direction of the force in the middle of the coil), the metal thing will move one way and then the other way, over and over again.

Most Google searches on solenoid just give you companies who produce them, but here's a webpage with instructions to build your own solenoid at home which will suck up a nail into its centre when turned on.

http://cdelker.tripod.com/electric/solenoid.html

Anyone who's taken physics, or an electricity and magnetism course, will know what a solenoid is. As for the pronunciation, we tend to say SOL-uh-noid (first syllable rhymes with ball) or SOLE-uh-noid (first syllable rhymes with hole) here.