In chemistry, an inert salt is a salt used to adjust the ionic strength of a solution. This is usually done in equilibrium or kinetic studies in order to reduce relative changes in the ionic strength of a solution. The real goal is to reduce changes in the activity coefficients of ionic species which allows the definition of conditional equilibrium or rate constants.
Any salt will affect the ionic strength, inert salts have the additional property that both the cations and the anions of the salt do or should not not interfere in any way with the molecules that are investigated. They are supposed to only influence the ionic strength.
Typical inert salts that are used include: NaClO, NaCl, KNO, NaNO, triflates (e.g. NaOSOCF). 
Inert salts are never perfectly inert and their use will always interfere with the process under investigation, although the influence may be negligible.