What exactly is an "oracle"? Wikipedia says that an oracle is a "blackbox", but I'm not sure what that means.
For example, in the Deutsch–Jozsa algorithm,
is the oracle just the box labeled $`` U_f " ,$ or is it everything between the measurement and the inputs (including the Hadamard gates)?
And to give the oracle, do I need to write $U_f$ in matrix form or the condensed form: $U_f$ gives $y \rightarrow y \oplus f(x)$ and $x \rightarrow x$ is enough with respect to the definition of an oracle?