The term petechia is derived from the Italian word petecchia, derived from the Latin word impetigo1. A petechia is basically a pinpoint "mini bruise"; a red-purple spot seen on the skin or mucous membrane caused by blood vessel leakage2. In other words, a small hemorrhage under the skin or mucous mebrane tissue. The term petechiae is the plural form of this word, and means several small dots. Larger areas of discoloration (larger amount of blood) are called purpura (up to 1 centimeter in diameter) and a large affected area is called an ecchymosis.
Petechia (and related purpura, ecchymosis) do not blanch (lose color) like an allergic rash will when light pressure is applied.
Petechiae are most often seen with bleeding disorders, but blunt trama injury is also a possible cause for the appearance of petechiae. Bleeding disorder refers to any disease where blood lacks normal clotting ability and "leaks" or hemorrhages into surrounding tissue. Possible causes of bleeding disorders (and resulting petechiae) include, but are not limited to: immune-mediated disorders, drugs, toxins, and medical treatments, such as radiation.
If your pet shows any signs of petechia without known trauma, it is advised that you contact your veterinarian as soon as possible to investigate the potentially serious causes for this sign to appear.