Gradients consist of continuously smooth color transitions along a vector from one color to another.
Once defined, gradients are then referenced by its 'id' attribute, using 'fill' or 'stroke' properties on a given graphics element: fill="url(#gradient)".
Further information:
<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewBox="0 0 100 100"></svg>