Unsustainable means impossible to continue or maintain over time. When something is unsustainable, it might work for a while, but eventually it will collapse, run out, or cause serious problems.

Think about a student who stays up until 2 a.m. every night playing video games. They might manage it for a few days, but soon exhaustion catches up with them. Their sleep schedule is unsustainable: they simply can't keep it up without getting sick or falling asleep in class.

In environmental discussions, the word often describes ways of using natural resources that can't continue forever. If a forest company cuts down trees faster than new ones can grow, that's unsustainable logging. The company might profit today, but eventually there won't be any trees left.

A business model can be unsustainable too. If a company spends more money than it earns every month, that's unsustainable: sooner or later, the money runs out. The opposite is sustainable, meaning something can continue indefinitely without causing harm or running out of resources.