Can be subtle...
Doctors have correctly explained that bipolar has two poles--depression
and mania (or hypomania
), unipolar one.
The subtlety is that the poles don't necessarily alternate. One may have 1, 2, ...Episodes of depression that look unipolar. Then a manic episode appears and looking back, those "uniplolar" episodes were really bipolar depressions. These require different treatment than unipolar.