To make or become hard or harder. The clay will harden if left out in the sun.
To make or become more severe or less sympathetic. The judge's attitude began to harden as the trial progressed.
To become more determined or resolute. His resolve to finish the marathon hardened with each passing mile.
To make or become more resistant to difficult conditions. The soldiers were trained to harden themselves against the harsh weather.
(of prices, markets, or rates) To rise or become firmer after a period of weakness. Share prices hardened during afternoon trading.