extrovert: an outgoing, socially confident person
introvert: a shy, reticent person
They are defined in terms of particular traits:
shyness: the quality or state of being shy
confidence: the feeling or belief that one can have faith in or rely on someone or something.
People can have differing degrees of each trait and differ in how they exhibit those in different social situations. However, by definition, if a person is predominantly shy and reticent in most social situations then we're justified in calling them an introvert.