What exactly is the Halo Effect?

What exactly is the Halo Effect?
A psychology textbook provides a "simplistic" definition of the Halo effect as a subjective bias about a person's one outstanding trait extending to influence the total judgment of that person.
E. L. Thorndike's 1920 paper titled "A Constant Error on Psychological Rating", published in Journal of Applied Psychology first documented this perception error (wahrnehmungsfehler) with regard to rating employees. This has also been followed up by Phil Rosenzweig's book on the same topic called The Halo Effect... and Eight Other Business Delusions That Deceive Managers.
Thorndike therein defined the halo effect as "a problem that arises in data collection when there is carry-over from one judgment to another."
He further expanded that it is "...an extension of an overall impression of a person (or one particular outstanding trait) to influence the total judgment of that person. The effect is to evaluate an individual high on many traits because of a belief that the individual is high on one trait. Similar to this is the 'devil effect,' whereby a person evaluates another as low on many traits because of a belief that the individual is low on one trait which is assumed to be critical."
So, to clarify, if possible, when an individual is found to possess one desirable trait, then that individual is automatically assumed to have many other desirable traits as well. A kind of an "angelic halo" surrounds the person, in the eyes of the beholder, and they can do no wrong. If a person is bestowed with good physical beauty, then this person is also presumed to possess a host of other positive attributes as well, such as social competence, intellectual competence, and personal adjustment.
The inverse phenomenon called the "Devil Effect," and sometimes the "Horn Effect", doesn't seem to get as much attention, even though its impact is just as prevalent in society. Here, if a person seems particularly deficient in a critical trait, then that person is automatically assumed to be deficient in many other traits as well, related or otherwise. For example, an employee who is constantly "late" to work (perhaps due to other non-work responsibilities in the morning) is assumed to be negligent in their work-related duties, not committed to the job/company/project, and perhaps even lazy overall.
Ultimately, these faulty biases may prove to become factual due to the Pygmalion effect, or "self-fulfilling prophecy ", further reinforcing future errors in perception due to bias and predisposition by the observer. The person working long hours (perhaps compensating for technical incompetence), assumed to be a good worker is given greater opportunity and thus attains greater, albeit undue, career advancement (cf: The Peter Principle). Conversely, the worker who dresses shabbily is assumed to care little about their job, and therefore bypassed for greater opportunity when the situation arises, regardless of suitability or capacity otherwise. Essentially, is phenomenon is a psycho-social application of the Law of Proximity, whereby certain unrelated observations, found in the comparable subjects in a narrow sample set, are assumed to have a high correlation, when, in fact, no such correlation exists.