How to define Nothingness?

Nothingness means complete emptiness.

absence of consciousness or life

lack of value, worth, meaning, etc.; uselessness, emptiness, insignificance, etc.

https://www.collinsdictionary.com/dictionary/engli...

https://www.britannica.com/topic/nothingness

blank sheet of paper or complete blackness

The space between breaths.

Nothingness is the absence of matter, energy, space, and time. It isn't possible to actually imagine total nothingness.

Birth and death

dont define

It is what it is

Melancholy is the closest thing I can compare to nothingness.

When you are in deep sleep without any dream, that is nothingness. But you cannot feel this when you are in the awakened state. Nothingness can be felt even when you have everything in your life. This is a spirit. The possession of this spirit can create this feeling. The reason for the possession of this spirit is your life situation. This is general. When you have lost a loved one either permanently or temporarily you can get this feeling.

The Void. The absence of anything at all. Nothingness does not exist at all. Only something exists.