A tundish is a funnel (or sometimes a small trough). It is used to make dripping water visible - say from a cistern overflow that cannot drip outside. The overflow is brought down inside and connected to a sink waste, but at eye level the overflow is broken into and a tundish inserted with the length from the cistern cut off 3" or 4" above the tundish. Thus you can see that the overflow is dripping, but it drips into the tundish and does not make a mess and it then drains away.
This question was one I posted about 3 months ago and was "closed" but somehow or other it seems to have resurrected itself. Isn't AB strange at times?