A '''silo''' is a tall round structure for storing grain.  A '''data silo''' is an informal, somewhat perjorative, term referring to a mass of data that is useable only by the original application that created it.

Even with such standards as XML -- designed to make data interoperable -- you're still ''practically'' limited by the application's implicit ontology which create a mini DomainSpecificLanguage which only the privileged converts can utilize.