Holding in a hit of cannabis from the bong or pipe so long that all the smoke is absorbed into the lungs. Upon exhaling, no smoke exits the body, as it has all been absorbed into the person smoking cannabis.
"Dude, he's held it in for 30 seconds!"
*guy smoking exhales, and no smoke exits*
"Damn man, you just did a ghost toke!"
"Fuck yeah I did!" :)