xmlns:xlink XLink Namespace Declaration

This is not an attribute, but the namespace pseudo-attribute. The value provides a URI to use for the XLink linking attributes (which has been set to a prefix of “xlink”). All namespace prefixes must be associated with a URL, and the prefix “xlink” has been set to the URL for the World Wide Web Consortium (W3C) XLink Recommendation.
FIXED (not settable) on many elements; click for list and usage
Value Meaning
http://www.w3.org/1999/xlink The prefix “xlink” has been set to the URL for the W3C XLink Recommendation.
Restriction This attribute has a fixed value which may not be changed in the XML document.
Tagged Sample

Inline graphic as a live link

...
<p><italic>Slow-front transient overvoltage (SFO)</italic>: 
 Transient overvoltage, usually unidirectional, with time to 
 peak 20 &#x3bc;s &lt; 
 <inline-formula>
  <alternatives>
   <tex-math notation="LaTeX">${T_p}$</tex-math>
   <inline-graphic xlink:href="1894_2015_mml_m1.eps"
     xmlns:xlink="http://www.w3.org/1999/xlink"/>
  </alternatives>
 </inline-formula> &#x2264; 5 000 &#x3bc;s, and 
 time&#xa0;to&#xa0;half&#xa0;value&#xa0;of&#xa0;a&#xa0;
 decreasing&#xa0;voltage 
 <inline-formula>
  <alternatives>
   <tex-math notation="LaTeX">${T_2}$</tex-math>
   <inline-graphic xlink:href="1894_2015_mml_m2.eps" 
     xmlns:xlink="http://www.w3.org/1999/xlink"/>
  </alternatives>
 </inline-formula> &#x2264; 20 ms.</p>
...
Related Resource
For a discussion on namespaces used in STS, see Namespacing NISO STS Elements.