Heterodoxy, as opposed to Orthodoxy, is the belief in something other (heteros) than the Orthodox Christian faith. In some senses, it is synonymous with heresy in that it is a departure from Orthodoxy, but it is distinct in that heresy by definition technically includes a specific choice to embrace and teach heterodox doctrine, usually accompanied by a formal anathema (condemnation) from a synod.
Heterodoxy may also be used to refer to all Christian confessions which are not the Orthodox Church.