Series = only the electric motor is connected to the drive train.
Prius is a "power split" or a "series parallel" hybrid, which is a bullshit term that means "we are special and not just a parallel hybrid"
In the modern colloquial terminology, slang usage seems to to be
"Series" = "range extended electric vehicle" or "generater in the trunk"
"Parallel" = "doesn't work without gasoline"
In general, people don't care HOW a series/parallel/power split/monkeyass hybrid works... they just care about the "doesn't work without gasoline" part, which is the tough pill to swallow with modern "totally not parallel" hybrids.