What Causes The Pond Ripple Effect In Field Of Vision ?

What causes me to see what i can best describe as a pond ripple effect in my field of vision? it can be round but not a smooth edge, or it can be more jagged. it usually starts as a small spot, then enlarges until it goes away. the line creates a distortion in my field of vision, but otherwise, i seem to feel fine. this happens occasionally and some times, for several days in a row then stops for a while.