How we can define liquefying in photoshop. I need it's definition only, so please be specific.

This is the Adobe definition of Liquify:

https://helpx.adobe.com/photoshop/using/liquify-filter.html#:~:text=The%20Liquify%20filter%20lets%20you,well%20as%20creating%20artistic%20effects.

I would say that Liquify is Photoshop's most powerful and most controllable tool for distoting images. 

Source: Adobe Help

https://helpx.adobe.com/photoshop/using/liquify-filter.html

The Liquify filter lets you push, pull, rotate, reflect, pucker, and bloat any area of an image. The distortions you create can be subtle or drastic, which makes the Liquify command a powerful tool for retouching images as well as creating artistic effects.

Hope that helps.

Pixel pushing and molding, based on the manipulation of the shape of an underlying mesh.