In chemistry, a reaction intermediate, or intermediate, is a molecular entity arising within the sequence of a stepwise chemical reaction. It is formed as the  reaction product of an elementary step, from the reactants and/or preceding intermediates, but is consumed in a later step. It does not appear in the chemical equation for the overall reaction.
For example, consider this hypothetical reaction:
:A + B → C + D
If this overall reaction comprises two elementary steps thus:
:A + B → X
:X → C + D
then X is a reaction intermediate.
The phrase itself, reaction intermediate, is very often abbreviated to the single word intermediate, and this is IUPAC's preferred form of the term. But this shorter form has other uses. It often refers to reactive intermediates. It is also used more widely for chemicals such as cumene which are traded within the chemical industry but are not generally of value outside it.