WSDL Definition Element

<definitions name="HelloWorldService"
   targetNamespace="https://www.dineshonjava.com/wsdl/HelloWorldService.wsdl"
   xmlns="http://schemas.xmlsoap.org/wsdl/"
   xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
   xmlns:tns="https://www.dineshonjava.com/wsdl/HelloWorldService.wsdl"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema">
............................................

<definitions>

From the above example we can conclude the followings points:

• The definitions element is a container of all the other elements.
• The definitions element specifies that this document is the HelloWorldService.
• The definitions element specifies a targetNamespace attribute. The targetNamespace is a convention of XML Schema that enables the WSDL document to refer to itself. In this example we have specified a targetNamespace of https://www.dineshonjava.com/wsdl/HelloWorldService.wsdl.
• The definition element specifies a default namespace: xmlns=http://schemas.xmlsoap.org/wsdl/. All elements without a namespace prefix, such as message or portType, are therefore assumed to be part of the default WSDL namespace.
• It also specifies numerous namespaces that will be used throughout the remainder of the document.

NOTE: The namespace specification does not require that the document actually exist at the given location. The important point is that you specify a value that is unique, different from all other namespaces that are defined.

References
Wikipedia for WSDL