The Zeldovich number is a dimensionless number which provides a quantitative measure for the activation energy of a chemical reaction which appears in the Arrhenius exponent, named after the Russian scientist Yakov Borisovich Zeldovich, who along with David A. Frank-Kamenetskii, first introduced in their paper in 1938. In 1983 ICDERS meeting at Poitiers, it was decided to name after Zel'dovich.
It is defined as
where 
* is the activation energy of the reaction
* is the universal gas constant
* is the burnt gas temperature
* is the unburnt mixture temperature.
In terms of heat release parameter , it is given by
For typical combustion phenomena, the value for Zel'dovich number lies in the range . Activation energy asymptotics uses this number as the large parameter of expansion.