Obstinacy is the quality of firmly sticking to your own viewpoint or desires. If you’ve ever watched a persistent two-year-old who wants something she’s been denied, you’ve witnessed obstinacy at its purest.
Adults show obstinacy too of course, when they refuse to be moved or change their mind. Obstinacy can also refer to a situation or trait that is hard to overcome, as in the obstinacy of a physics problem that will not be solved or molasses that will not be stirred. Obstinacy is a noun for obstinate behavior — it can also be called bullheadedness, pigheadedness, willfulness, and plain old stubbornness.