along an axis

Axes are defined for arrays with more than one dimension. A 2-dimensional array has two corresponding axes: the first running vertically downwards across rows (axis 0), and the second running horizontally across columns (axis 1)

>>>print np.arange(12)
>>>print np.arange(12).reshape(3, 4)
>>>print np.arange(12).reshape(3, 4).mean(axis=1)
>>>print np.arange(12).reshape(3, 4).mean(axis=0)