SAT Solution Space - Definition of Cluster of Solutions
I think a cluster of solutions is a maximal set of solutions $T$ s.t. you can reach every $\tau' \in T$ from every other $\tau \in T$ by a sequence of solutions $\{\tau_i\}_{0\leq i\leq n}$ ($\tau = \tau_0$ and $\tau' = \tau_n$) where the hamming distance between each consecutive pair of solutions is bounded, i.e. they are just connected components in the graph where two solutions are adjacent iff the hamming distance between them is less than the bound.

See these notes by Dimitris Achlioptas (or papers on statistical physics and random k-SAT).