Smoke that stayed in your body after you exhale, that comes up anytime in a cough or burp anytime after finished smoking.
Really all it is, is sometimes when take a big enough rip, smoke might stay in your system (lungs stomache) even though you exhaled your hit. Sometimes your body will completely absorb this excess smoke, but other times your body will make you burp or cough up this extra smoke. It can happen really any time after you smoke. Sometimes it happens a few minutes after smoking, other times it might happen a half hour after smoking.
Your friend burps up a ridiculous ammount of smoke 10 minutes after the bowl you two smoked was cashed. You look at him and say "that was the biggest white ghost I have ever seen".