title: In NLP/probability/ML notation: what does a tilde over a letter mean?
body: <p>I am reading <a href="https://dl.dropboxusercontent.com/spa/juxddtvfvlf51yv/paper/convex.pdf" rel="nofollow noreferrer">this</a> paper. In section 1.1 he says:</p>
  
  <p><img src="https://i.stack.imgur.com/1hbSM.png" alt="enter image description here"></p>
  
  <p>What do the tildes above the letters mean? How can I translate this sentence into ordinary English?</p>