A ghostload is the 'invisible' load that comes out of a climaxing penis when one has masturbated or had sex more then a certain amount of times in a row. Usually about the 5th or 6th time in a two hour timespan.
Since the testicles can only produce so many sperm per day, your balls eventually runs out of it when you orgasm too often without resting properly.
Although blowing a ghostload doesn't feel like a real accomplishment for the participant(s), it does however have the advantage of no afterwards clean-up.
Friend 1: I had sex with Jimmy's mom so many times yesterday that at the end of the night I only blew ghostloads over her face.
Friend 2 : That's awesome.
Friend 1: Yes.
John: Dude! What the hell are you doing jerking off over my sleeping grandma!!
Matt: Jeez, relax man, it's only a ghostload.
John: Oh, sorry, my bad.
Matt: no problem, high five!