Reticence
Part of speech: noun
Definition: the avoidance of saying too much a silent nature
Example sentence: Often confused with shyness, introversion does not imply social reticence or discomfort. Rather than being averse to social engagement, introverts become overwhelmed by too much of it, which explains why the introvert is ready to leave a party after an hour and the extravert gains steam as the night goes on.
Taciturnity
Part of speech: noun
Definition: The trait of being taciturn.