Heat stroke is a condition arising from extremely high body temperature (rectal temperature of 105 to 110 degrees Fahrenheit), which leads to nervous system abnormalities (such as lethargy, weakness, collapse or coma). Abnormally high body temperature (also called hyperthermia) develops after increased muscular activity with impaired ability to give off heat due to high heat and humidity or respiratory obstruction.
It is more common in dogs but can occurs in cats as well.
To prevent heatstroke in dogs, NEVER EVER leave them in the car. Make sure they are in a well ventilated area at all times and out of the direct sun. If you are uncomfortable in that area or room, your dog probably is too. Also, make sure your dog always has access to plenty of fresh clean water and shade.