The <defs>
element is used to store graphical objects that will be used at a later time. Objects created inside a <defs>
element are not rendered directly. To display them you have to reference them (with a <use>
element for example).
Graphical objects can be referenced from anywhere, however, defining these objects inside of a <defs>
element promotes understandability of the SVG content and is beneficial to the overall accessibility of the document.