Forchheimer spots are a fleeting enanthem seen as small, red spots (petechiae) on the soft palate in 20% of patients with rubella. They precede or accompany the skin rash of rubella. They are not diagnostic of rubella, as similar spots can be seen in measles and scarlet fever.
The sign is named after Frederick Forchheimer.
References
Forchheimer spots Wikipedia(Text) CC BY-SA