What, exactly, is head cheese?

[sawyl]

From yesterday's Guardian, this piece by Lynsey Hanley on working at Greggs provoked a fit of barely-suppressed giggling on the bus home, not least because it mentions what to do with spilled head-cheese:

One area that necessitated apprenticeship was the meat-slicing machine, on which rounds of pressed ham, tongue, beef and brawn – the elderly betting man's favourite, otherwise known as head-cheese – were slapped for shaving off by the quarter. Once I hauled a new ham out of the fridge only for it to slip out of my hands and slide across the floor, landing at the manager's feet. "Oh, just wash it down," she advised, which I did while my customer eyed the oven for fresh bloomers.