Unknown symbol/expression in text about adaptive filters (cst)
The standard normalized step-size LMS algorithm computes the current step-size according to

$$ \mu = \frac{c}{s_k^T \cdot s_k}  $$

where $c$ is a suitable scale factor and $s_k^T \cdot s_k$ is the total energy of the current tap inputs. The algorithm aims to adjust step size according to input signal power; when input has large power then decrease the step-size so that filter coefficient update will not deivate much. But if the signal goes to zero or very small levels (silence durations, or small variance sections), then the division will possibly be very large and algorithm may produce unbounded (very large) outputs or even diverge...

To prevent this from happening, a small nonzero factor; the regularisation factor, is added to the denominator 

$$ \mu = \frac{c}{s_k^T \cdot s_k + \delta} $$

to limit the maximum of the fraction to $c/\delta$ when input power goes to zero.