title: Big-O notation's definition
body: <p>I really want to know the real definition. I have tried to read a book, but couldn't understand it.</p>
  
  <p>O: Big-O notation worst case.<br>
  Θ: Theta notation average case.<br>
  Ω: Omega notation best case.</p>
  
  <p>Why does Wikipedia represent the speed of algorithms just in Big-O including its average, best and worst cases? How come they did not replace with those formal keywords?</p>