Complex sentence: Introverts are easily overwhelmed by too much stimulation from social gatherings and engagement, introversion having even been "defined" by some in terms of a preference for a quiet, more minimally stimulating external environment.
Split into simpler sentences: Introverts are easily overwhelmed by too much stimulation from social gatherings. Socializing is not the only kind of stimulus that can overwhelm introverts, however; introversion has been defined by some authors as a preference for a quiet, more minimally stimulating external environment.