Seal point is a term used to describe a cat's coloring. It means that the cat has a cream or fawn colored body, but dark brown points (face, paws, and tail). 'Seal', in this case, refers to the dark brown color.
Seal point is most often applied to Siamese cats, who often have this coloring. Other breeds, and mixed breeds, can also have this coloring.
Seal pointing in cats is caused by an odd allele of the gene that activates hair pigmentation. In these cats it is only activated at low temperatures. This means that hair follicles on the trunk of the cat, receiving full body temperature, do not express the gene, but those on the extremities are cool enough to become colored. If you were to bandage a foot or ear, the higher temperature would cause it to lighten in color over time. Likewise, cold weather and illness can also cause changes in body temperature and fur color.
Related terms
Seal-Lynx Point: a seal point with underlying tabby coloring.
Seal-Tortie Point: very dark points, with tortoiseshell elements.
Seal isn't the only color of point, only the most common. You might also find chocolate point, blue point, smoke point and lilac point cats.