What is the scientific definition of Introversion/Extroversion? Personally I believe introvert and extrovert are one of the most thrown around and misused words. I would go as far to say as what they mean in common everyday English is not the same as what they mean in psychology. 

What exactly is the definition of introversion and extroversion? I've seen lots of psychology surveys, such as this one, where the possibles answers are too limited. I mean, I know a lot of people who would be considered extroverts but are still tired after a music festival or concert.
I totally agree. These words have become colloquial and have lost much of their meaning - and are therefore increasingly less helpful. Introversion is confused with shyness and sensitivity, while Extroversion is confused with social skills. This is partly because there is no agreed upon definition, insofar that they are operationally defined by each psychometric test/framework differently. That is to say, the same theoretical construct/concept is measured differently in each test, and therefore are operationally defined by a different set of responses.

Its is generally agreed that is is a continuum and that individuals do not reside exclusively on the same part of the continuum all the time, across contexts (which is where your festival example comes from). That is, an individual may have a strong/weak tendency but may behave differently dependent upon the situation they find themselves in. Anecdotally, for what it's worth, I find an individual's tendency is best revealed when they are under pressure or are experiencing anxiety.

For the original interpretation, look no further than Carl Jung's definition. The most widely accepted operational definition according to most Psychology research is possibly The Three Factor Model, while the most popular operational definition, where most lay people hang their hat is the Myers-Briggs Type Indicator. I'd say people warm to this the most, and is the definition most people are likely referring to when they speak of 
Introversion/Extroversion. 

Looking to these three sources, you should get the original theoretical definition for the construct.