One equivalent: In reaction stoichiometry, the amount of one substance that reacts with one mole of another substance. This will often (but not always) be a 1:1 mole ratio.

In this S_N2 reaction, one mole of CH₃Cl reacts with one mole of NaI to produce one mole of CH₃I and one mole of NaCl. One equivalent of CH₃Cl reacts with one equivalent of NaI.

2 eq. H2 Pt

In this catalytic hydrogenation reaction, one mole of acetylene reacts with two moles of H₂ to produce one mole of ethane.

1 eq. H2 Pt

1 eq. H2 Pt

Reaction of acetylene with one equivalent of H₂ gives ethylene; reaction of acetylene with two equivalents (2 eq.) of H₂ gives ethane.