Recessed lighting:

This is any lighting where the main body of the light does not extend beyond the ceiling, wall or floor in which it is located.

Down lights:

Usually refers to recessed GU10 or MR16 light fittings.

Pendant light fitting:

This is your most common light fitting, with a ceiling rose at the top (round plastic box with the electrical connections), a lamp holder at the bottom and a cable connecting the two together.

Lamp:

Correct name for a light bulb.

B22 / Bayonet:

Possibly the most common type of light used in pendant lamps.

E27 / Edison screw:

This is a screw based lamp, used more commonly in table lamps.

E14 / SES / Small Edison screw:

A small screw based lamp, used with golf ball and candle shaped lamps.

GU10:

Typically used in down lights, mains voltage. These fittings have 2 mushroom shaped pins, the need to be twisted a quarter turn to be fitted.

MR16:

These look the same as a GU10 in shape but have 2 straight pins – when being fitted they push directly into the lamp holder. These lights are low voltage (12V) and require a transformer to work.

Halogen lamp:

A halogen lamp is an incandescent lamp with a tungsten filament contained within an inert gas. Typically used in the form of GU10 and MR16 down lights, but a wide range is now available.

LED lamps:

Light Emitting Diode – these are used in lamps to replace standard incandescent lamps. LED’s are extremely efficient and can use 90% less energy than standard incandescent lamps.

Lighting transformer:

Used for low voltage, reducing mains voltage down to 12V. Different transformers can take varying amounts of fittings depending on the load size.

Gang:

Refers to the number of switches, sockets or other items in a single unit.
E.g. A double socket is called a 2 gang socket or 2g socket.

One way switch:

One switch controlling one light or one group of lights.

Two way switch:

Two switches in separate locations, both operating the same light or group of lights.

Intermediate switch:

These are put into a two way switched system when additional switches are needed, so you can have three or more switches controlling the same light or set of lights.
E.g. In a long hallway, or stairs covering multiple floors.

FCU switched and unswitched (Spur):

Fused connection unit. This is a fuse carrier – it's an alternative to having a plug and usually used to connect fixed electrical appliances such as a dishwasher, washing machine (etc.) to a mains supply. It has many uses and can have an integrated switch allowing you switch off the connected circuit.

Main earth:

This is the earthing that your whole property is connected to - it connects your consumer unit to the energy companies mains supply.

Earth bonding:

These are green and yellow cables that are connected from your consumer unit to your incoming gas, water and any other supply, along with any other exposed metal structures. This is so that in the event of a fault current connecting to any conductive part, the circuit breakers will trip, cutting power to the circuit.

Miniature circuit breaker – this is an over current protection device. This works in the same way as a fuse cutting power when the current reaches a given level, though unlike fuses they can be reset.

Residual Current Device – this is a safety device most commonly found in a consumer unit. It is a circuit breaker that trips when there is a difference between the amount of current on the live side and the neutral side.

This unit contains both an RCD and MCB, it does the job of both devices.