The targetNamespace attribute

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.cafeconleche.org/schemas/greeting/"
  xmlns="http://www.cafeconleche.org/schemas/greeting/"
>
 
  <xsd:element name="GREETING" type="xsd:string"/>

</xsd:schema>