In British English, a brand name (and eponym) for a type of small gas water heater, usually serving a single tap (sometimes with a spout emerging directly from the heater itself) and ignited from a pilot light when that tap is run. Has provided copious hot water and occasional carbon monoxide poisoning to countless dodgy bedsits, student houses and other low-cost housing, although electric versions are now becoming more common.
The Ascot brand (made by the Ascot Gas Water Heater Ltd) proper was set up by one Bernard Friedman who introduced the heaters into Britain from the USA in 1932. For probably the majority ofthe housing stock in the country in the mid-20th century it was the first source of "on tap" hot water - previously water would have been heated on the fire or kitchen range.
Should you be looking at places to live or stay, bear in mind that an Ascot could be a bit more expensive to run than a system with a boiler or immersion heater and a tank (although more flexible if your hot water needs vary a lot) and make doubly sure that the heater (and any other gas appliance) has proper, unobstructed ventilation.
Synonyms: geyser (English midlands and north - thanks to wertperch)