The deductible.
On insurance policies, the deductible is an amount that the individual must pay of their own expenses before the insurance policy covers any costs. After meeting the deductible amount, most insurance policies then have a ratio of how they will pay expenses -- for instance, 80% paid by the insurer, 20% by the individual. For some sorts of expenses (listed in the policy) insurers will cover the whole amount.
Since you asked this question in the history area, let's note a little history. The first health insurance sort of coverage in the United States was accident insurance, offered by a company founded in 1850 that covered persons injured in railroad or steamboat accidents. General health insurance covering sickness and such didn't begin in the US until after 1890. Today, we've come to expect health insurance as a part of everyone's life and budget -- indeed, the Affordable Care Act of 2010 made it a legal requirement for Americans to carry health insurance or else pay a tax penalty if they did not. But health insurance wasn't always such a common commodity.
