Wikipedia provides an initial place to look for a definition of psychopathy:
Psychopathy is traditionally a personality disorder characterized by persistent antisocial behavior, impaired empathy and remorse, and bold, disinhibited, and egotistical traits. It is sometimes considered synonymous with sociopathy. Different conceptions of psychopathy have been used throughout history that are only partly overlapping and may sometimes be contradictory.
From this we can expect historical definitions that are potentially contradictory. However, it is a recognized personality disorder although under other names:
Hervey M. Cleckley, an American psychiatrist, influenced the initial diagnostic criteria for antisocial personality reaction/disturbance in the Diagnostic and Statistical Manual of Mental Disorders (DSM), as did American psychologist George E. Partridge. The DSM and International Classification of Diseases (ICD) subsequently introduced the diagnoses of antisocial personality disorder (ASPD) and dissocial personality disorder (DPD) respectively, stating that these diagnoses have been referred to (or include what is referred to) as psychopathy or sociopathy. The creation of ASPD and DPD was driven by the fact that many of the classic traits of psychopathy were impossible to measure objectively. [my emphasis]
That professionals consider the traits of psychopathy "impossible to measure objectively" suggests that any definition should be based on antisocial personality disorder (ASPD) or dissocial personality disorder (DPD).
Wikipedia describes ASPD and DPD together as follows:
Antisocial personality disorder (ASPD or APD) is a personality disorder characterized by a long term pattern of disregard for, or violation of, the rights of others. A low moral sense or conscience is often apparent, as well as a history of crime, legal problems, or impulsive and aggressive behavior.
Antisocial personality disorder is defined in the Diagnostic and Statistical Manual of Mental Disorders (DSM). Dissocial personality disorder (DPD), a similar or equivalent concept, is defined in the International Statistical Classification of Diseases and Related Health Problems (ICD), which includes antisocial personality disorder in the diagnosis. Both manuals provide similar criteria for diagnosing the disorder.
Wikipedia contributors. (2019, July 10). Antisocial personality disorder. In Wikipedia, The Free Encyclopedia. Retrieved 11:03, August 3, 2019, from https://en.wikipedia.org/w/index.php?title=Antisocial_personality_disorder&oldid=905686596
Wikipedia contributors. (2019, July 29). Psychopathy. In Wikipedia, The Free Encyclopedia. Retrieved 10:28, August 3, 2019, from https://en.wikipedia.org/w/index.php?title=Psychopathy&oldid=908418525