A metropolitan is a senior bishop ranking above ordinary bishops and, in most Eastern churches, below a patriarch, originally the bishop of the chief city of a province.
Bishop of the mother city
The title derives from the bishop of the metropolis, the chief city of a Roman province, who came to hold a precedence and a limited oversight over the other bishops of the province, so the metropolitan is the senior bishop of a province, presiding over its bishops and its common affairs while each bishop governs his own diocese. As with the other senior titles, the office is a rank of jurisdiction built upon the order of bishop, not a further ordination.
East and West
In the Eastern churches the metropolitan is a standard rank in the hierarchy, above the ordinary bishops and, where there is a patriarch, below him, and in some Orthodox churches the metropolitan heads an autocephalous or autonomous church, so the exact standing varies. In the Western church the corresponding office is that of the archbishop who presides over a province, and metropolitan and archbishop denote much the same function, the senior bishop of a province, in the two halves of Christendom. Entry is by appointment or election to the see that carries the rank. The metropolitan thus represents the senior provincial bishop of the Eastern churches, the counterpart of the Western archbishop, holding a precedence and a limited oversight over the bishops of his province while remaining, in order, a bishop among bishops.