Exponential linear unit (ELU), like Leaky ReLU, has a small slope for negative values. But instead of having a straight line, it has a log curve, as shown in the following diagram:

It can be expressed as follows:

The ELU function is implemented in Python as follows:

def ELU(x,alpha=0.01):
    if x<0:
        return ((alpha*(np.exp(x)-1))
    else:
        return x