Constancy (noun)

1. The quality of being constant, unchanging, or persistent in time.
2. The degree to which a quantity or quality remains unchanged or reliable.

Origin:

Late middle english: from old french constante, from latin constantia, from constant- 'standing firm', from the verb constare.

Examples:

1. The constancy of her support meant a lot to him.
2. The constancy of the sun's position allowed them to determine their location at sea.
3. The constancy of the light source is crucial in this experiment.
4. Her constancy in her beliefs won her many admirers.
5. The constancy of the supply chain is essential for the production of goods.