In the reaction between HCl and NaOH, the compound that is used up is HCl. This is because HCl is a strong acid and NaOH is a strong base, and when they react, they neutralize each other to form water and salt. The chemical equation for this reaction is:

HCl + NaOH → NaCl + H2O

In this equation, HCl and NaOH are the reactants, and NaCl and H2O are the products. The reaction proceeds by the transfer of a hydrogen ion (H+) from HCl to NaOH, resulting in the formation of NaCl and H2O. Therefore, HCl is the compound that is used up in this reaction.