Secondary plasma or plasma gas are neutral (semi-)stable gas particles, which are generated by a plasma discharge after recombination.
Physical plasma is by definition an excited gas. It consists of radicals, charged atoms and molecular fragments, as well as excited molecules. As long as the energy is sufficient, this state is maintained. If the energy gets too low, the species recombine very fast to uncharged, but sill reactive molecules. This state according to the definition no plasma but merely a gas mixture and is therefore called secondary plasma.