無常 is the state of being “not permanent,” “not enduring,” transitory, or evolving.

It can also mean variable or changeable. In some contexts, it can refer to a ghost that is supposed to take a soul upon death. Following that, this term can also mean to pass away or die.

In the Buddhist context, this is a reminder that everything in this world is ever-changing, and all circumstances of your life are temporary.
If you take the Buddhist philosophy further, none of these circumstances are real, and your existence is an illusion. Thus, the idea of the eternal soul is perhaps just your attachment to your ego. Once you release your attachment to all impermanent things, you will be on your way to enlightenment and Buddhahood.

Language notes for this word when used outside the context of Buddhism:
In Korean Hanja, this means uncertainty, transiency, mutability, or evanescent.
In Japanese, the definition orbits closer to the state of being uncertain.

無常の風 is an old Japanese proverb that means the wind of impermanence or the wind of change in Japanese.

This can refer to the force that ends life, like the wind scattering a flower's petals. Life is yet another impermanent existence that is fragile, blown out like a candle.

The first two characters mean uncertainty, transiency, impermanence, mutability, variable, and/or changeable.

In some Buddhist contexts, 無常 can be analogous to a spirit departing at death (with a suggestion of the impermanence of life).

The last two characters mean “of wind” or a possessive like “wind of...” but Japanese grammar will have the wind come last in the phrase.