This meaning refers to someone who shows great persistence and determination, often in overcoming challenges or pursuing goals. It can describe individuals who do not easily give up.
This meaning indicates a person who is set in their ways and resistant to change or influence. It can describe someone who refuses to listen to reason and insists on their own opinions.
Lactic-acid bacilli are such unyielding bacteria that, even if frozen, they would simply hibernate and can restore themselves as if they came back to life.