Runaway includes methane feedback emissions- especially the enormous stores of Arctic carbon as methane emitters. Carbon dioxide and nitrous oxide feedback GHG sources are also huge. In runaway all three would be self reinforcing and accelerating.
Methane has a global warming effect of 86X CO2 for 20 years after emissions.
Methane having increased over two and a half times since industrialization stalled in 200 but since 2006 it has been on a renewed sustained increase- that became explosive from 2020 and the scientists say this increase is due to warmed wetland methane feedback emissions. The methane they think is coming mainly from tropical and some from northern wetlands
It seems most of the methane is being emitted from warming tropical wetland but the largest increase is from Far North wetland peat.
The highest atmospheric methane on the planet is recorded at East Trout Lake, Saskatchewan Canada Lake right on the southern edge of Canada's vast wetlands at 2010 ppb.
This use of the world runaway is not a scientific term but the basis in science is definite.
This 'runaway' means the situation caused by positive (bad) feedbacks in which global warming accelerates the rate of global warming totally beyond any capacity of human control.
When describing this situation the scientists may use the terms of rapid global warming and abrupt global climate change.
Tipping points, irreversible impacts, and singularities are also scientific terms that apply