title: What is "Dirty" in React.js?
body: <p>I am trying to understand React.js and often come across a term "Dirty" like 
  dirty checker/checking, dirty data ,dirty model</p>
  
  <p>I followed <a href="https://stackoverflow.com/questions/21109361/why-is-reacts-concept-of-virtual-dom-said-to-be-more-performant-than-dirty-mode">this question</a> but could not make out what actually the term <strong>Dirty</strong> convey &amp; why we call it dirty.</p>