Gaku

Gaku o Gakuboricould be translated as "frame". 

This is the name of the predominantly black background surrounding or framing the main elements in Japanese tattoos.

It began to be used in the 1820's, just when the Japanese tattoo was called "Japanese tattoo". horimono.  

It is usually composed of representations of the elements of nature. Mostly wind, clouds and waves are used. to symbolize the elements of air and water, and representations of rocks and flowers to represent the earth element are also common. Fire appears as is, not as frequently as the others, and often associated with divinities, or magical creatures or objects.