(Science: radiobiology) fusion reaction products trapped in a plasma. Ash is bad because (a) it generally radiates more strongly than the fuel ions, and thus reduces energy confinement, and (b) it creates additional plasma pressure and/or reduces pressure available for fuel ions.
Controlling ash is a major area of fusion research. Ideally one would be able to extract the ash ions after diverting an appropriate fraction of their energy to heating the fuel ions, and then convert the remaining ash energy to electricity.
current research involves using rf waves to transfer energy from ash ions to fuel ions, and to push the ash into the scrape-off layer, where it can be collected via divertors.