Partner, copartner, colleague, ally, confederate all denote an associate but they differ markedly in connotation and are not freely interchangeable.
Partner implies especially an associate in a business (partnership ) or one of two associates (as in some games, in a dance, or in marriage).
Since partner alone implies association, the addition of co- , with its implication of association, in copartner sometimes adds little or nothing to partner; thus, one may refer equally to partners or copartners in crime. Copartner, however, can distinctively imply fellow partner or equality of share.
Colleague applies typically to an associate in office or in professional or academic relations.
Ally and confederate, though referable to persons, more frequently denote an associated state or government.
Ally suggests an often temporary association in a common cause (as the prosecution of a war) or in affairs of policy or statecraft.
Confederate (see also CONFEDERATE 2 ) implies an entering into a confederacy or confederation and usually suggests a closer or more permanent union for strength and solidarity.