Liquid

A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material.

A liquid's shape is determined by the container it fills. That is to say, liquid particles (normally molecules or clusters of molecules) are free to move about the volume, but they form a discrete surface that may not necessarily be the same as the vessel. The same cannot be said about a gas; it can also be considered a fluid (things that flows, including gas), but it must conform to the shape of the container entirely.