What does ||= mean?

Possible Duplicate:
What does ||= (or equals) mean in Ruby?
What does ||= mean?

I have just started learning RubyMotion and in a lot of examples I see the ||= syntax. What does this mean?

Here is an example:

def window
  @window ||= begin
    w = UIWindow.alloc.initWithFrame UIScreen.mainScreen.bounds
    w.rootViewController = @navigationController
    w
end

It is difficult to search symbols, google ignored the symbols in my query.