A 10,000 BTU air conditioner has 10,000 British Thermal Units of cooling capacity, or energy production required to remove heat from a room. This is a medium-sized capacity unit. Window air conditioners normally range from 5,000 BTU to 24,000 BTUs.
Estimating 20 BTUs for each square foot of living space, a 10,000 BTU air conditioner might be suitable for a 500-square-foot room. There are other factors that come into play, such as the height of the ceiling, the size of windows and doorways, how many people regularly occupy the room, sun exposure, and if the room is a kitchen or bedroom.