(covers information from several alternate timelines)
Entropy was a thermodynamic quantity representing the unavailability of a system's thermal energy for conversion into mechanical work.
In an alternate 2379, The Doctor exposed Kes to a biotemporal field in order to push her cells back to an earlier stage of entropic decay and thereby extend her life. Instead it reactivated dormant chronitons in her body, taking her out of temporal sync and back in time. (VOY: "Before and After")