A vehicle for the conveyance of the wounded from where they are (and probably don't want to be) to the hospital (where they still probably don't want to be). Thus far has consisted of everything form your basic horse drawn hay cart to a sleek and sexy helicopter, though in its most popular form is a van/truck (sorry, lorry) with something resembling a bed and a CMT or Paramedic in the back. See also MEDEVAC.
If one were to fall victim to a wah, then one might need the services of a Waaaahmbulance. I'll get my coat.
In The Beginning
In the good old bad old days, when you got sick the local medicine man would come round to your hut/shanty/slum and do his best to bless/blood let you right there on the spot. With the beginnings of the industrial age we started getting sick and hurting each other on an appropriately industrial scale and started having to make our own bloody way to help (see Monty Python and the Holy Grail).
Pride can be taken that ambulances are almost without a doubt a child of the army; Credit most frequently being given to our favourite Corsican Corporal, Napoleon. It was actually his MO, but who's counting? Not so keen with the idea of a downed soldier having to make his own way back to the hospital, or more than likely laying out in the mud until someone less busted up than he decided to give him a hand, Napoleon's Doc organized teams of horse drawn carts to go out, give limited treatment to, and return wounded soldiers to the hospital after a battle. It should be noted however that these same carts were the same ones also used to collect the dead and as modern medicine was anything but at this point, a second ride wasn't out of the question.
Things carried on pretty much the same technology wise though the American Civil War. Improvements were however made on the organizational front with the formation of dedicated ambulance companies, Field Hospitals and systems of triage and casualty evacuation. Around this time, large cities started getting in on the ambulance gig as well. The City of London is credited with operating ambulances for the removal of small pox victims as early as 1867. Most likely based on hearses of the time, these units came equipped with the latest in removable cot technology. It is also worth noting that morphine and brandy were also standard issue. As is the case today however, nobody rode for free.
Those in the funeral business also started to catch on that their hearses could fit a bed just as well as a box. Thus, many early and indeed relatively recent ambulances services were operated by funeral homes. I'm sure there are more than a few cases of a patient expiring en route and the "ambulance" bypassing the hospital and going right back to the funeral parlour.
Out of beer, more to follow.