unnatural

An unnatural thing is one that doesn't occur naturally or isn't normal for the world we live in.

When people say something is unnatural, they often mean it's not like what usually happens. For example, if a tree grows in the middle of a city, it might seem unnatural because trees normally grow outside cities where there's more space and air to breathe. If someone tells you that it's unnatural for humans to live on another planet, they're saying it's not something that naturally happens. This word can also be used when people talk about things that are not part of the normal way of doing things, like a car flying through the air (which is usually reserved for airplanes) or someone being able to breathe underwater without any equipment.