Block sensitivity ($\mathrm{bs}(f)$)

For an input $x$, count the largest family of pairwise disjoint nonempty sets of coordinates such that flipping each set individually changes the Boolean function value. Maximizing over inputs gives block sensitivity. Sensitive sets need not be contiguous and need not consist of single bits. The standard bounded-error quantum query complexity lower bound is $\Omega (\sqrt{\mathrm{bs}(f)})$. A certificate for $f(x)$ meets every sensitive set, so its size bounds their disjoint count.