What does “|=” mean? (pipe equal operator)

I tried searching using Google Search and Stack Overflow, but it didn’t show up any results. I have seen this in opensource library code:

Notification notification = new Notification(icon, tickerText, when);
notification.defaults |= Notification.DEFAULT_SOUND;
notification.defaults |= Notification.DEFAULT_VIBRATE;

What does “|=” ( pipe equal operator ) mean?