In The Sacred and the Profane, Mircea Eliade defines the sacred as that which is not non-sacred.
This is a tautology, albeit a useful one: that which is sacred is an interruption in the non-sacred world. The sacred provides identity, induces prosperity and is an authority to appeal to; it is a relationship of power. Examples include sacred rocks (ie, the stone at Delphi supposedly swallowed by Chronos), the burning bush and the incarnation of God in Jesus Christ.
The sacred stone remains a stone, and a sacred tree remains a tree, but these and other ordinary objects that are deemed sacred are adored because they are sacred; they have become something else yet still remained themselves, for they continue to participate in the cosmic milieu. An event where the sacred manifests itself is a hierophany.