SVG: Defining Elements with an ID within a defs
Element
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.