Negative Feedback, as applied to Mafia, is when a faction that is winning finds it more difficult to keep winning.
Positive feedback tends to be more common in most setups. However, there is a minor element of negative feedback in many cases. A Town that is successfully lynching scum has few scum to find in a larger playerlist, making it easier for the remaining scum to hide; on the other hand a Mafia that has killed the Town up to LyLo comprises almost half the pool of suspects and must outtalk the Town to continue surviving.
Specific plays may carry a parcel of negative feedback; for example a successful bus will nail a scumbag now but may hinder the Town's ability to find the busser later.