title: Console says "$ is not defined" on a basic jquery code
body: <p>This is my code which doesn't work:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>$(document).ready(function() {
   $(document).on('click', '.item', function() {
    $(this).width(120);
   });
  });</code></pre>
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.item {
   width: 100px;
   height: 100px;
   border-radius: 10px;
   background-color: red;
   margin-right: 10px;
   display: inline-block;
   cursor: pointer;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;!DOCTYPE html&gt;
  &lt;html&gt;
  &lt;head&gt;
   &lt;title&gt;jQuery&lt;/title&gt;
   &lt;link rel="stylesheet" type="text/css" href="style.css"&gt;
  &lt;/head&gt;
  &lt;body&gt;
  &lt;div class="item"&gt;&lt;/div&gt;
  &lt;div class="item"&gt;&lt;/div&gt;
  &lt;div class="item"&gt;&lt;/div&gt;
  &lt;div class="item"&gt;&lt;/div&gt;
  
  &lt;script type="text/javascript" src="script.js"&gt;&lt;/script&gt;
  &lt;/body&gt;
  &lt;/html&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>In the browser console it says "$ is not defined" as soon as the page loads. What is the problem?</p>