Numbers made up for a particular example that have no meaning beyond being placeholders for the given example. That is, numbers which have been pulled "out of your ass".
"This example shows how the cache lines are organized, but all these addresses are assnumbers."
by Mr Punch December 09, 2008
a random number - one that's pulled right out of the ass
No, I didn't calculate the sale price - it's an ass number
by Carl Barringer May 30, 2007