Gases and plasmas have neither definite shapes nor definite volumes. They both expand to fill available space, and can be reshaped by their containers. Liquids have definite volume, while solids have both definite shape and definite volume.
Plasma is a type of ionized gas, and therefore shares many of the same physical, if not chemical, properties of gases. Gases have neither definite shape nor definite volume because of the limited attraction between their particles. This allows gas and plasma particles considerable freedom of movement. In contrast, particles in liquids and solids are attracted to each other, and therefore are much less able to move to fill an available space or, in the case of solids, to change their shape.