A welt (also known as a wheal or weal) is a type of lesion: a section of skin raised in a ridge or bump, and reddened. The reddening comes from a widening of cappillaries, and the ridge comes from a build-up of extracellular fluid (lymph) (but not "filled" with fluid as a blister). A welt can be caused by a blow or lash to the skin, by an allergic reaction, or by an injection with a hypodermic syringe.
Welts in spanking
Welts due to spanking activity can be slow to fade. If they break open or bleed they may be subject to infection.