Cooperativity is not only a phenomenon of ligand binding, but also applies anytime energetic interactions make it easier or more difficult for something to happen involving multiple units as opposed to with single units.  (That is, easier or more difficult compared with what is expected when only accounting for the addition of multiple units).  For example, unwinding of DNA involves cooperativity:  Portions of DNA must unwind in order for DNA to carry out replication, transcription and recombination. Positive cooperativity among adjacent DNA nucleotides makes it easier to unwind a whole group of adjacent nucleotides than it is to unwind the same number of nucleotides spread out along the DNA chain.  The cooperative unit size is the number of adjacent bases that tend to unwind as a single unit due to the effects of positive cooperativity.  This phenomenon applies to other types of chain molecules as well, such as the folding and unfolding of proteins and in the "melting" of phospholipid chains that make up the membranes of cells.  Subunit cooperativity is measured on the relative scale known as Hill's Constant.