In chemistry, a stressor is something that either speeds up a reaction rate or keeps the reaction rate the same. Stressors include light, temperature and elevated sound levels. Stressors also include the phenomena of substance concentration (does not shift equilibrium), catalysis, substance surface area (speeds up the reaction rate), and the nature of the reactants.

In accordance with Le Chatelier's Principle, reactions shift away from stressors. For example, see the following equilibrium reaction. There are three distinct species: CO, H₂ and CH₃OH.

If the concentration of any of these species is changed, the reaction will shift accordingly. If [CO] is increased, the reaction will progress towards the products. If [CH₃OH] is increased, the reaction will progress towards the reactants.

• Equilibrium is a dynamic process with forward and reverse reactions occurring simultaneously. When the two are in balance, the reaction appears complete.
• Reaction rate describes how rapidly a chemical change takes place.

In organismal biology, a stressor is an external state change (such as a change of temperature or salinity) that results in a physiological response from an organism required to maintain homeostasis. Many organismal traits are occult until revealed by observing responses to stressors; this is particularly true of microorganisms.