What is the difference between a psychopath and sociopath? The Internet is of course full of memes from Sherlock Holmes show, based on one of the episodes having Sherlock self-diagnose:


  I'm not a psychopath, I'm a highly functioning sociopath


But what exactly is the difference?

Wikipedia does not actually seem to offer any meaningful distinction between the two, including seemingly indicating that they are synonymous in DSM but not actually named there.


  The DSM and International Classification of Diseases (ICD) subsequently introduced the diagnoses of antisocial personality disorder (ASPD) and dissocial personality disorder, stating that these have been referred to (or include what is referred to) as psychopathy or sociopathy.
Ironically enough, Wikipedia does offer as meaningful a distinction as any of the answers here so far:

The term sociopathy may have been first introduced in 1909 in Germany by biological psychiatrist Karl Birnbaum and in 1930 in the US by educational psychologist George E. Partridge, as an alternative to, or a subtype of, the concept of psychopathy.[137] It was used to indicate that the defining feature is violation of social norms, or antisocial behavior, and has often also been associated with postulating social as well as biological causation.[139][140][141][142]
There are various contemporary usages of the term. Robert Hare, who may believe that biological factors are predominant in causing psychopathy,[143] claimed in a 1999 popular science book that sociopathy and psychopathy are often used interchangeably, but in some cases the term sociopathy is preferred because it is less likely than is psychopathy to be confused with psychosis, whereas in other cases which term is used may &quot;reflect the user's views on the origins and determinates of the disorder&quot;. Hare contended that the term sociopathy is preferred by those that see the causes as due to social factors and early environment, and the term psychopathy preferred by those who believe that there are psychological, biological, and genetic factors involved in addition to environmental factors.[78] Hare also suggests another possible distinction: he defines psychopathy as not having a sense of empathy or morality, but sociopathy as only differing in sense of right and wrong from the average person.[144][145][verification needed][bolding added for emphasis]

While this largely echoes the existing answers, it includes references to published books and articles, involves the idea's history, and gives the impression that the distinctions between psychopathy and sociopathy are largely in the mind of the authors. They vary somewhat from one author to the next, and don't seem to have much empirical support as distinct diagnoses.
One also ought to question the utility of the proposed distinctions. Secondary psychopathy is also meant to imply social etiology, but also lacks empirical support somewhat. Defining a disorder by violation of social norms is a particularly poor approach to abnormal and clinical psychology, because social norms are typically subjective and ill-defined themselves, and do not necessarily apply outside a contemporary cultural context. Such a definition must possess very limited reliability across time and space.

References
78. Hare, R. D. (1999). Without conscience: The disturbing world of the psychopaths among us. New York: Guilford Press.
137. Rutter, S. (2007). The psychopath: Theory, research, and practice (pp. 37). New Jersey: Lawrence Erlbaum Associates.
139. Partridge, G. E. (1930). Current conceptions of psychopathic personality. The American Journal of Psychiatry, 1(87), 53–99.
140. Felthous, A., &amp; Sass, H. (2008). International handbook on psychopathic disorders and the law, Vol. 1. Wiley.
141. Hildebrand, M. (2005). Psychopathy in the treatment of forensic psychiatric patients: Assessment, prevalence, predictive validity, and clinical implications. Rozenberg Publishers.
142. Partridge, G. E., &amp; Hamblin Smith, M. (1930). Epitome of current literature: Current conceptions of psychopathic disorder. The British Journal of Psychiatry, 76, 838. Retrieved from http://bjp.rcpsych.org/content/76/315/838.1.extract.
143. Ronson, J. (2011). The psychopath test (pp. 225). Riverhead Books.
144. Hare, R. (2006). Snakes in suits: When psychopaths go to work. New York, NY: Harper Collins Publishers.
145. Skilling, T.A., Harris, G. T., Rice, M. E., &amp; Quinsey, V. L. (2002, March). Identifying persistently antisocial offenders using the Hare Psychopathy Checklist and DSM antisocial personality disorder criteria. American Psychological Association.