To become hard (tough, resistant to pressure).
To make something hard or harder (tough, resistant to pressure).
To strengthen.
To modify (a website or other system) to make it resistant to malicious attacks.
To become or make (a person or thing) resistant or less sensitive.
To become or make (a consonant) more fortis.
The process by which something hardens.
That which hardens, such as a material used for converting the surface of iron into steel.