title: What is ivar in Rails controller?
body: <p>I can see <code>ivar</code> term mentioned many times here <a href="https://github.com/rails/rails/issues/18950#issuecomment-77924771" rel="noreferrer">https://github.com/rails/rails/issues/18950#issuecomment-77924771</a> but can't find straight forward explanation what is the iVar.</p>
  
  <p>What is it and can this term be used outside controller context?</p>