Geometrically frustrated spin systems often do not exhibit long-range magnetic ordering, resulting in either quantummechanically disordered states, such as quantum spin liquids~1, or classically disordered states, such as spin ices~(2,3) or spin glasses~4. Geometric frustration may play a similar role in charge ordering~(5,6), potentially leading to unconventional electronic states without long-range order; however, there are no previous experimental demonstrations of this phenomenon. Here, we show that a charge-cluster glass evolves on cooling in the absence of long-range charge ordering for an organic conductor with a triangular lattice. A combination of timeresolved transport measurements and X-ray diffraction reveals that the charge-liquid phase has two-dimensional charge clusters that fluctuate extremely slowly (<10–100 Hz) and heterogeneously. On further cooling, the cluster dynamics freezes, and a charge-cluster glass is formed. Surprisingly, these observations correspond to recent ideas regarding the structural glass formation of supercooled liquids~(7–10). Glassy behaviour has often been found in transition-metal oxides, but only under the influence of randomly located dopants~~(11,12). As organic conductors are very clean systems, the present glassy behaviour is probably conceptually different.
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