Elimination is a type of organic reaction in which one molecule is split into two fragment molecules. It is the reverse of addition reaction, in which two fragments of a molecule eliminate as a small molecule as a result a double (or triple) bond is formed in the organic molecule. Simple examples include:
CH3CH2Br + NaOH à CH2=CH2 + HBr, CH3CH2OH + H2SO4 + heat à CH2=CH2 + H2O
First reaction is called dehydrobromination, the second one is dehydration, after the molecule that is being eliminated. Most eliminations follow second order kinetics, meaning rate of reaction is dependent upon the concentrations of the reactant as well as the base, and hence called E2 type of elimination reaction. However, there are some examples of first order eliminations, called E1. The type of pathway that is predominant is dictated by certain physical factors like nature of the substrate, base, solvent used etc.