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A fluid is a substance that can flow. It ultimately assumes the shape of the containing vessel because it cannot withstand shearing stress. Thus, both liquids and gases are fluids.

Important characteristics of fluids:

(i) The atoms or molecules in a fluid are arranged in a random manner.

(ii) A fluid cannot withstand tangential or shearing stress for an indefinite period. It begins to flow when a shearing stress is applied.

(iii) A fluid has no definite shape of its own. It ultimately assumes the shape of the containing vessel. So a fluid has no modulus of rigidity.

(iv) A fluid can exert/withstand a force in a direction perpendicular to its surface. So a fluid does have a bulk modulus of rigidity.