Which personality disorder gets its name from the outdated concept that the individual with the diagnosis exists somewhere between the psychotic and the "neurotic" planes of functioning?

The correct answer is borderline personality disorder.

Borderline personality disorder (BPD) refers to a mental disorder than significantly affects one's ability to manage and cope with emotions. BPD usually manifests in an individual's interpersonal relationships. BPD sufferers typically display the following symptoms:

i) Emotional instability
ii) Intense fear of abandonment and serious efforts to avoid it
iii) Unstable interpersonal relationships
iv) Impulsivity
v) Unstable and poor self-image or self-esteem