Brain freeze- Rapid cooling and rewarming of the capillaries in the sinuses. When ice cream hits your palate, the temperature is sensed by nearby pain receptors, which signal the brain via the trigeminal nerve, one of the major facial area nerves. The brain interprets this facial area pain as coming from the forehead. To relieve pain, some doctors suggest drinking a warm liquid or pressing the tongue against the roof of the mouth to warm the area.
I learned this definition while looking at an online article. I had been curious about the true explanation for brain freeze ever since I first felt it, which has happened since childhood. I thought someone else would like to know.