A reaction intermediate or simply intermediate is a molecule that is formed during a chemical reaction. It is not the final product, but it is something that is more like the product than the reactants. After every step in a reaction mechanism, an intermediate is formed.
Intermediates usually stay around for a little time because they are very reactive. However, it is important not to confuse an intermediate with a transition state. The latter is at a point of maximum energy. An intermediate instead is at a point of minimum energy, i.e. it is a stable molecule. Intermediates can be isolated from a reaction if needed.
For example, in the reaction:
A + B → X → C + D
X is the intermediate. In going from A + B to X and from X to C + D, the reaction will go through a transition state.