Tribal is the name given to solid black graphic designs which have become popular in recent years.
Its origins are, among others, in the Polynesian islands where the tribes have a tradition of tattooing for ritual and ceremonial purposes; the Maori of New Zealand with their facial mokos are probably the most well known example.
Modern tribal designs do not generally have any symbolic meanings and are simple body decorations for those who are unsure of what kind of picture they could live with. Celtic designs, gangland style lettering and Japanese characters all come under this description in modern tattooing.
Tribal combines well with the other styles mentioned and can be useful as background or for connecting separate pieces.