Contextual explanation: Sudden emotional response to the unexpected. 
Aback is best understood as an emotional state—specifically, the feeling of being startled, surprised, or caught off-guard by something unforeseen or unexpected. It describes a momentary reaction where one's emotional equilibrium is disrupted by a sudden event or revelation.

Themes: Surprise and emotional disruption. 
Aback is closely tied to the theme of surprise, highlighting how individuals respond emotionally to unexpected circumstances. This theme is relevant in contexts where rapid adaptation or reassessment is required due to unforeseen developments.

Nuance: Startled reaction. 
What distinguishes aback within its category is the immediacy and involuntary nature of the response—it is not a sustained mood but a brief, often visible, reaction to surprise. This nuance sets it apart from other emotional states that may be more enduring or internally processed.