The voltage between the plates of a capacitor
That's a somewhat confusing way of putting it. It's related to the direction you define as "the opposite direction"; here's a diagram.







simulate this circuit &ndash; Schematic created using CircuitLab

The arrows here always point from negative to positive. The left schematic shows how you might think of it as "the same direction"; the right one shows how you might think of it as "the opposite direction". Note that these two schematics are identical.