Corn starch and water make this kind of liquid. Under pressure it acts like a solid but in normal pressure it acts like a liquid. What is it called?

It is a Non-Newtonian Fluid.

Remember SILLY PUTTY? That gloopy sticky stuff that could remove newsprint but would BOUNCE when thrown at the floor. That is also a non-Newtonian Fluid but I do not believe that it is a colloidal suspension !

A non-Newtonian fluid is a fluid in which shear stress is not simply proportional solely to the velocity gradient, perpendicular to the plane of shear. Non-Newtonian fluids may not have a well-defined viscosity.

Principle types of non-Newtonian fluid include:
plastic, pseudo-plastic, perfectly plastic, Bingham plastic, dilatent, rheopectic, thixotropic & visoelastic.

[From uiuc.edu 2004 article, no longer online]

You can find another Physics Van question and answer about non-newtonian fluids here. As far as I know, there is no such thing as a non-newtonian solid. The cornstarch and water mixture you were asking about is conisdered a non-newtonian fluid, but it is actually composed of both a solid and a liquid. This kind of mixture is called a colloidal suspension, and you can read more about it in the other answer.

Really interesting stuff to read about:
https://en.wikipedia.org/wiki/Non-Newtonian_fluid
non-Newtonian fluid is a fluid in which shear stress is not simply proportional solely to the velocity gradient, perpendicular to the plane of shear. Non-Newtonian fluids may not have a well-defined viscosity.

Principle types of non-Newtonian fluid include:

plastic
pseudo-plastic (apparent viscosity reducing with rate of shear. E.g. gelatine, blood and liquid cement)
perfectly plastic (strain does not result in opposing stress)
Bingham plastic
dilatent (apparent viscosity increasing with rate of shear. E.g. concentrated solution of sugar in water)
rheopectic (apparent viscosity increases with duration of stress. E.g. some lubricants)
thixotropic (apparent viscosity decreases with duration of stress. E.g. non-drip paints and tomato ketchup)
visoelastic (having both viscous and elastic properties. E.g. bitumen and nylon)