In chemistry and biology, catalysis is the acceleration (increase in rate) of a chemical reaction by means of a substance, called a catalyst, which is itself not consumed by the overall reaction. The word is derived from the Greek noun κατάλυσις, related to the verb καταλύειν, meaning to annul or to untie or to pick up.
A catalyst provides an alternative route of reaction where the activation energy is lower than the original chemical reaction. Catalysts participate in reactions but are neither reactants nor products of the reaction they catalyze. An exception is the process of autocatalysis where the product of a reaction helps to accelerate the same reaction. They work by providing an alternative pathway for the reaction to occur, thus reducing the activation energy and increasing the reaction rate. More generally, one may at times call anything that accelerates a reaction, without itself being consumed or changed, a "catalyst" (for example, a "catalyst for political change").
A good example of a catalyst is in the disproportionation of hydrogen peroxide. Hydrogen peroxide reacts to give water and oxygen gas by itself:
2 H₂O₂ → 2 H₂O + O₂
Usually, this reaction is slow. On the addition of manganese dioxide to a dilute solution of hydrogen peroxide, an effervescence is observed, and much oxygen, detectable by a glowing splint, is evolved. The manganese dioxide may be recovered, and re-used indefinitely, thus it is a catalyst — it is not consumed by the reaction.
A promoter is an accelerator of catalysis, but not a catalyst by itself. An inhibitor inhibits the working of a catalyst.

Definitions

Main article: catalytic cycle Catalytic cycles
Catalysts work by providing an (alternative) mechanism involving a different transition state and lower activation energy. The effect of this is that more molecular collisions have the energy needed to reach the transition state. Hence, catalysts can perform reactions that, albeit thermodynamically feasible, would not run without the presence of a catalyst, or perform them much faster, more specific, or at lower temperatures. This can be observed on a Boltzmann distribution and energy profile diagram. This means that catalysts reduce the amount of energy needed to start a chemical reaction.
Catalysts cannot make energetically unfavorable reactions possible — they have no effect on the chemical equilibrium of a reaction because the rate of both the forward and the reverse reaction are equally affected (see also thermodynamics). The net free energy change of a reaction is the same whether a catalyst is used or not; the catalyst just makes it easier to activate.
The SI derived unit for measuring the catalytic activity of a catalyst is the katal, which is moles per second. The degree of activity of a catalyst can also be described by the turn over number (or TON) and the catalytic efficiency by the turn over frequency (TOF). The biochemical equivalent is the enzyme unit.

Catalysts and reaction energetics
Catalysts can be either heterogeneous or homogeneous. Biocatalysts are often seen as a separate group.
Heterogeneous catalysts are present in different phases from the reactants (for example, a solid catalyst in a liquid reaction mixture), whereas homogeneous catalysts are in the same phase (for example, a dissolved catalyst in a liquid reaction mixture).

Types of catalysts

Main article: Heterogeneous catalysis Heterogeneous catalysts

Main article: Homogeneous catalysis Homogeneous catalysts

Main article: Biocatalysis Biocatalysts
In the context of electrochemistry, specifically in fuel cell engineering, various metal-rich catalysts are used to promote the efficiency of a half reaction that occurs within the fuel cell. One common type of fuel cell electrocatalyst is based upon tiny nanoparticles of platinum which adorn slightly larger carbon particles. When this type of platinum electrocatalyst is in contact with one of the electrodes in a fuel cell, it increases the rate of the redox half reaction in which oxygen gas is reduced to water (or hydroxide or hydrogen peroxide).

Electrocatalysts
Catalysis is of paramount importance in the chemical industry. The production of most industrially important chemicals involves catalysis. The earliest commercial processes are the Haber process for ammonia synthesis and the Fischer-Tropsch synthesis. Research into catalysis is a major field in applied science, and involves many fields of chemistry, notably in organometallic chemistry, and physics. Catalysis is important in many aspects of environmental science, from the catalytic converter in automobiles to the causes of the ozone hole. Catalytic, rather than stoichiometric reactions are preferred in environmentally friendly green chemistry due to the reduced amount of waste generated.

Significance
Estimates are that 60% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.
Manganese dioxide is used in the laboratory to prepare oxygen by the decomposition of hydrogen peroxide to oxygen and water.
Some of the most famous catalysts ever developed are:
Some examples of (famous) catalysts that perform specific transformations on functional groups:
These given examples show that different catalysts perform other transformations on the same functional groups, where the reaction would not run, run very slowly, or not run in a specific manner without the presence of the catalyst.
The most effective catalysts are usually transition metals or transition metal complexes.

Catalytic converters made from platinum and manganese break down some of the more harmful byproducts of automobile exhaust.
the Haber process for the synthesis of ammonia from nitrogen and hydrogen, where ordinary iron is used as a catalyst.
Transformations of olefinic groups:

• the Ziegler-Natta catalyst used to mass produce polyethylene and polypropylene.
  the Grubbs' catalyst for olefin metathesis. Notable examples
  While transition metal catalysts are well established, a new trend is toward organocatalysis which use comparatively simple organic molecules as catalysts. While typically, catalyst loading is much higher than transition metal-based catalysts, the catalysts are usually commercially available in bulk, helping to reduce costs drastically. Organocatalysts of the "new generation" are competitive to traditional metal-containing catalysts and are owing to low product inhibion applicable in substoichiomertric quantities. The chemical character of organocatalysts offers new and attractive perspectives and advantages to synthetically working chemists.
  
  Catalytic processes
  
  The journal Catalysts and Catalysed Reactions
  Autocatalysis
  Enzyme
  Enzyme catalysis
  Phase Boundary Catalysis
  SUMO enzymes