The equivalent weight of an element is the mass which combines with or displaces 1.008 gram of hydrogen or 8.0 grams of oxygen or 35.5 grams of chlorine. These values are the atomic weight divided by the usual valence. For oxygen, as an example, that is 16.0g / 2.
Equivalent weight has the dimensions and units of mass, unlike atomic weight, which is dimensionless. The equivalent weight of a compound can be got by dividing the molecular weight by the number of positive or negative electrical charges carried by its ions (which result from dissolving the compound).