The best replacement for reactant is reagent.

The best alternatives to reactant are rated as following:
★★★★★  reagent
★★★★  product
★★  substrate
  product is at equilibrium
  limiting reagent

Your alternative for reactant is not listed? Suggest!
Speaking about reactant, one probably shouldn't forget about percent yield.

Other search terms related to reactant and their ratings are:
★★★★★ percent yield
★★ theoretical yield
★ excess of the other reactant
You have in mind a companion for reactant and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of reactant in all years. Click on graph for more.
reactant
reagent
product