A security or commodity is fungible if it is perfectly interchangeable with any other of the same type and class.
Most financial securities are fungible: a share in a particular company is exactly the same as another share in the same company (of the same class if the company has more than one class of share).
Financial markets for commodities usually specify the nature of the commodity (e.g the purity of a metal being traded) sufficiently tightly for it to be treated as fungible.
As prefect subsutitutes for each other, fungible goods have an infinite cross price elasticity.