(noun)
the process of something becoming hard, stiff, or tough or an instance of this,, especially of this happening to a part of a living organism;
e.g. The friction led to the hardening of the surface as it developed a protective coating.
(verb)
becoming harder or causing something to become harder;
e.g. If you tough the plaster while it is hardening, it might lose its shape.
(verb)
becoming figuratively harder and more severe, becoming less sympathetic, or causing something/someone to become this way;
e.g. Difficult experiences have a way of hardening a person and making them tougher.
(verb)
becoming more steady or clearly defined, or causing something to become this way;
e.g. Their findings were hardening into a well-defined theory. The stock prices were hardening at their highest point.