Negative feedback occurs when the output of a system

System

System is a set of interacting or interdependent components forming an integrated whole....

 acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback

Feedback

Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

 of the system is negative, then the system will tend to be stable.
In many physical and biological systems, qualitatively different influences can oppose each other. For example, in biochemistry, one set of chemicals drives the system in a given direction, whereas another set of chemicals drives it in an opposing direction.