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A quantum spin liquid is a phase of interacting quantum spins whose ground-state behavior avoids conventional magnetic order through quantum fluctuations and can exhibit long-range quantum entanglement and fractionalized excitations. Absence of a measured average magnetization alone is insufficient: thermal disorder or averaging over ordered states can also give a zero average.

An exactly solvable example is the honeycomb model with bond-dependent $XX$, $YY$ and $ZZ$ interactions. Its solution expresses the spin problem using fermions coupled to a discrete gauge field and yields phases whose excitations are anyons. Such a model makes fractionalization precise, beyond the informal picture of spins merely continuing to fluctuate.^[ref]