Symbolism.

Symbolism is the representation of an image that expresses a certain feel or notion to that picture. For example with the following. The hooded figure who often has a sickle is almost always manifested into death, a figure who comes for you when you pass on. That is mostly what is depicted with that symbol. The crazy lines with a circle in the middle, would not be known to anybody had it not been completely transformed into the bio hazard sign. This picture is known throughout the world as a symbol for danger of toxic waste, etc. Finally the celtic picture represents eternity and never ending hence the lines keep going, it also shows progress and cycles.