> ... if a person is predominantly shy and reticent in 
    > most social situations then we're justified in calling 
    > them an introvert.

Sure, just understand that you're using the colloquial definition of "introvert" (a synonym for "shy") and not the technical definition, as developed for the Meyers-Briggs Personality Type Indicator:

    "People who prefer extraversion draw energy from 
    action: they tend to act, then reflect, then act
    further. If they are inactive, their motivation 
    tends to decline. To rebuild their energy, extraverts 
    need breaks from time spent in reflection. Conversely,
    those who prefer introversion "expend" energy through 
    action: they prefer to reflect, then act, then reflect
    again. To rebuild their energy, introverts need quiet
    time alone, away from activity."

[1] http://en.wikipedia.org/wiki/Myers-Briggs_Type_Indicator#Att...