Matter is defined as anything which has volume and mass. Three phases of matter: Solid, liquid and gas.
Solid: The substances in which particles are closely packed, has definite volume and definite shape.
Liquid: Particles are loosely packed as comparison to solid, has definite volume and take the shape of container i.e. indefinite shape.
Gas: Particles are loosely packed, has indefinite volume and indefinite shape.
Thus, gas: variable volume; variable shape
liquid: fixed volume; variable shape
solid: fixed volume; fixed shape