NumPy is an open source package (i.e. extension library) for the Python programming language originally developed by Travis Oliphant. It primarily provides

N-dimensional array data structures (some might call these tensors...) well suited for numeric computation.

sophisticaed "broadcasting" operations to allow efficient application of mathematical functions and operators over entire arrays of data, e.g. calling sin(x) an NumPy array x returns the elementwise sine of the numbers contained in x.