title: Definition and exact function of ?attr/selectableItemBackground
body: <p>I stumbled upon the expression <code>?attr/selectableItemBackground</code> through <a href="https://stackoverflow.com/questions/19714682/android-apply-selectableitembackground-in-xml-with-support-v7">android - apply selectableItemBackground in xml with support v7</a>.<br>
  I would like to look into the exact functionality of that expression, since I do not understand what the question mark at the beginning symbolizes and how exactly it accomplishes its task.</p>
  
  <p>It says that it is part of the support library v7, but I tried looking it up and couldn't find helpful insight into</p>
  
  <ul>
  <li>what exactly it <em>is</em>,</li>
  <li>where it is defined and</li>
  <li>what precisely it does.</li>
  </ul>