The dual of the SVM without offset: \[ \max_{\alpha_1,\dots, \alpha_n} \sum_{i=1}^n \alpha_i - \frac{1}{2}\sum_{i,j=1}^n \alpha_i \alpha_j k(x_i,x_j) \quad \mbox{s.t.}\quad 0 \le \alpha_i \le C,\, \forall i=1,\dots,n \quad \quad \quad (QP)\]
Decomposition
Optimizing \((QP)\) one \(\alpha_i\), or a few \(\alpha_i\)s, at a time.
Working set selection
The set \(I \subseteq \{1,\dots, n\}\) of dual variables that the decomposition optimization algorithm consider at a given time.
Shrinking
During optimization using decomposition method, a dual variable may converges to \(\alpha_i = 0\) or \(\alpha_i = C\) and stays there. Such a dual variable can be taken out of the optimization.
From Cross Validated: What Are Shrinking Heuristics?
See section 5.1 of the official documentation of LIBSVM.