An oracle is a "black box" operation (function) that is used as an input to an another algorithm (for example Deutch-Jozsa, Grover etc.). A parameter or feature of the Oracle is infered by the algorithm.

Following is taken from Microsoft oracles:

Often, such operations are defined using a classical function $f:\{0,1\}^n \rightarrow\{0,1\}^m$ which takes an $n$-bit binary input and produces an $m$-bit binary output.

One can also go through this question.