WordNet sense 1 (the atomic weight of an element that has the same combining capacity as a given weight of another element; the standard is 8 for oxygen):

eq, combining weight, equivalent weight

WordNet sense 2 (equal in amount or value):

like, same

WordNet sense 3 (being essentially equal to something):

tantamount

WordNet sense 4 (well matched; having the same quantity, value, or measure as another):

equal