Astronomical waste is a term introduced by Nick Bostrom for the opportunities we're losing out on by not colonizing the universe.
The universe is vast. There are many galaxies, each containing many stars. In a future with space colonization, each star could support a large population of people leading worthwhile lives. During any given year, we irrecoverably lose an amount of energy that could have powered a civilization like ours for many billions of years.
The prospect of advanced technology only makes the numbers more extreme. Such technology would make it possible to support far more, and better, lives with the same resources.
Bostrom notes that in a wide range of moral theories — in particular, those based on linear aggregation of value — considerations of astronomical waste outweigh all others.
If so much potential is lost every year, one could conclude that we should start colonization as soon as possible. But since the amount of resources available in total is much larger still than the amount lost in a year, a better utilitarian prescription is to minimize the risk of losing out on space colonization entirely. Bostrom calls this prescription "maxipok", for the maximum probability of an OK outcome.