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Suprathermal is an adjective to describe particles (suprathermal particles) with more kinetic energy than they would have if merely set in motion by the material's heat: particles within a gas or plasma that do not fit a Maxwell-Boltzmann distribution and comprise a more substantial tail in the distribution toward the more energetic (suprathermal tail). They are identified statistically: the thermal radiation of any temperature can produce particles up to ultrarelativistic speed, but the measured distribution of a substantial sample can indicate a high probability of the presence of suprathermal particles. Such particles may travel further (on average) than would particles moving solely due to the material's thermal energy.
The presence of suprathermal particles has been identified in solar wind, sometimes associated with a solar eruption. The generation of such energetic particles is still under study: they might have migrated from some hotter region, e.g., from shock heating, and/or a magnetic field or electric field may be involved, producing suprathermal charged particles, i.e., suprathermal ions and/or suprathermal electrons.