What is a Pilot Light?

Pilot lights are an indicator that all the doors are proved closed. They are the ones inside the Driver's cab, as shown in Chris M's post, and in the C stock,

it is the little blue lamp at the end of the door open buttons. I think the orange lights on the outside are called Door Fault Indicators?

You can see one in action on the DLR, it's the white "ADC" ("all doors closed" I presume) light on the control panel by each door. Surprisingly I've not managed to find a picture of one online, so I'll take a picture next time I'm on the DLR (likely tomorrow).

ISTR that there is a much more esoteric explanation, as the original pilot lights were not blue.

They also used to be in the guard's position inside the trains. The signal to proceed could not be given until the blue light indicated all doors were closed.

Also, the whole 'pilot light' system can be overridden if the fault can't be found on any of the circuits controlling it or the doors are closed and still no 'pilot light' is obtainable. The train has to be taken out of service with all doors physically checked by the driver to ensure they are closed and then the seal is broken and the cut-out operated. I don't know what they are called on other stock but our '72's have a 'Door Interlock Cut Out Switch. If a door/set of doors remain stubbornly open then a member of staff is posted by those doors to ensure no one tries to board as it passes along the line. A couple of months ago at Piccadilly Circus THREE doors/sets could not be closed so three members of staff were needed! Station staffing levels as they are thesedays, the station had to close as it went under 'minimum numbers' and could only re-open again once they'd all got back!!

I'm not sure it's a nickname, on a 62TS guard's panel at least it is officially labelled as such by cast-in lettering, and is referred to as a pilot light on all the official documentation such as drawings and parts lists

"Pilot" has many related meanings that indicate something that is always there in case of emergency or as a guide or indicator.
Pilot light
Pilot signal
Station Pilot
Maritime Pilot.....

no, the switches are on the door actuating mechanisms and make when the doors are fully closed

ROD = Re-open Doors
CTD = Close This Door
COD = Close Other Doors

IIRC from what I was last told

(edit: I don't know why my brain turned the O into an A in the acronym for the last one - fixed )

Cheers!

Could be I suppose, but why would it lock with an operators key? Taking the whole panel off wouldn't normally be done outside the depot would it?
On the Stratford International emergency excercise, the doors were opened and unlocked by a switch in a cupboard adjacent to the articulation iirc.

I guess it depends on the level of fault-finding that the Train Captains are expected to do in the event of a problem and also on what is behind the panel. I'm thinking of UndergrounD train seats which contain DICs, TUPIs and even ELs which a Train Op may need as part of their fault-finding procedure.