A large cross-shaped mark that appears on or near the forehead of an anime or manga character; the shape is supposed to represent the throbbing of veins in anger. Often used with the super deformed or "SD"-style of art, it is typically used in somewhat light-hearted scenes to indicate extreme anger at a situation. (Contrast this to anger in a serious scene, which typically wouldn't resort to such markings.)
A vein-throb is sometimes accompanied by a stylized "hellfire" background behind the character, or four-pointed yellow stars in the character's eyes, if he/she is really angry. (For example, Tsuyoshi-kun in Kodomo no Omocha is shown with all three of these when he gets mad and goes on a rampage.)
Other stylized expressions used to show emotion in anime and manga include the sweat-drop, the facefault, and the nosebleed.