A fluid is a substance which cannot preserve its own shape..
A fluid is a material substance which cannot sustain a shearing stress when it is at rest.

A fluid can be either a liquid or a gas.   A liquid retains a set volume and has a surface of separation above which is a vapour and below which is a liquid.   A gas, whatever its quantity, when placed in an empty closed vessel will fill the vessel completely.  It is important to note that a gas is generally considered to be an compressible fluid and a liquid is generally considered to be incompressible.  In practice a liquid is compressible but generally very large pressures are required to cause quite small changes of volume.