A synod is a meeting between the hierarchs of a particular Church to decide a matter. It is a Greek word usually applied in the East; I guess the meetings the USCCB has would technically constitute synods - a clearer example is if all Latin bishops assembled to discuss a matter.
Now the qualitative difference between an ecumenical council and a synod is disputed - a Latin will say essentially any synod convened by the Pope is an ecumenical council. Byzantines would probably say an ecumenical council is the convening of a general synod between all Apostolic Churches - hence in many Eastern [read: Byzantine] texts will refer to the Seven Holy Councils (i.e. the councils held between the 4th and 8th centuries). The Oriental Churches, on the other hand, accept the first three councils as ecumenical.
The disagreement of what constitutes ecumenical isn’t a dispute of truth, rather if the council itself is qualitatively a regional synod (i.e. convened to address administrative matters of a locale) or a general synod convened to state a universal, infallible statement. The fact that the Latin Church includes all their local synods in their count of ecumenical councils is amusing, and I’m sure someone will take offense at that statement but I don’t care.