Willpower (意志力, "Ishi-ryoku") is a persons ability to refrain from being manipulated by another person's Intent or their ability to gather strength or energy from sheer desire similarly to Rock Lee who was capable of standing up to fight Gaara despite being unconscious. The Will of Fire's ability to grant one the ability to gather chakra and strength where there is none is also an example of willpower in action.
Certainly, here's a scale of willpower levels from 1 to 6, with 1 being the lowest and 6 being the highest:
Tier 0/5: Feeble[]
Individuals at this level possess very little willpower. They are easily influenced or manipulated by others and often lack the determination to overcome challenging situations.
Tier 1/5: Weak[]
People in this category have a bit more willpower than the feeble, but they still struggle to stand firm against external influences. They may give in to pressure relatively easily.
Tier 2/5: Average[]
This is the standard level of willpower that most individuals possess. They can resist manipulation to a reasonable extent and gather some strength through sheer determination when needed.
Tier 3/5: Strong[]
Individuals with strong willpower can resist most attempts at manipulation and draw upon their inner resolve to face challenges. They're known for their determination and resilience.
Tier 4/5: Exceptional[]
At this level, willpower is a formidable trait. People with exceptional willpower are nearly impervious to external manipulation, and they can muster significant strength even when faced with dire circumstances.
Tier 5/5: Indomitable[]
This is the pinnacle of willpower. Those with indomitable wills are virtually unbreakable. They are incredibly resistant to manipulation, and their ability to gather strength through sheer desire is awe-inspiring. They can face the most daunting challenges with unwavering determination.