What is the name for the percentage of the cost that the patient must pay?

A person walks into a UniVista Insurance company office where people are signing up for health care plans under the Affordable Care Act in Miami.

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They might think they know a lot about health insurance. They're wrong — and their ignorance could cost a lot of money.

A new survey finds that just 4 percent of Americans are able to correctly define all four terms that determine how much they would personally have to pay for medical services and drugs they receive under their health insurance plans.

The same PolicyGenius survey also found wide gaps between consumers' perceived and actual knowledge of each of those four terms — deductible, co-pay, coinsurance and out-of-pocket maximum — as people routinely overestimated their understanding of those words.

On average, the percentage of consumers able to correctly define one of those terms was 25 percentage points less than the percentage who said they could define the term.

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And the survey of more than 2,000 insurance consumers found that despite that overconfidence about terms relating to out-of-pocket costs, half the population said it is only "somewhat confident" in their ability to select the right health insurance plan for their needs — with 12 percent of them saying they are not confident "at all."

Jennifer Fitzgerald, CEO and co-founder of PolicyGenius, said the findings — which come as millions of Americans prepare to sign up for 2017 health coverage — are sobering because they put consumers at a big potential financial disadvantage from picking the wrong insurance plan, or not understanding their exposure to charges when they seek treatment.

"The most significant result is a very high bill that you weren't expecting," said Fitzgerald, whose digital company helps consumers compare different types of insurance and provides guidance on which plans to pick.

"If you don't understand how a deductible works, how coinsurance works ... [when] you come in for a procedure or a lab test, those are the kinds of things that can result in a bill for $1,000 or $2,000," she said.

Fitzgerald, whose company has just launched its first purchasing decision-making app for health insurance buyers, said it's not surprising that many people don't know the definitions of certain insurance terms, or are not completely confident in their ability to pick the right health plan.

"It's quite possibly one of the most complicated decisions anyone would have to ever encounter," she said.

In recent years, a growing number of insurance plans have increased the out-of-pocket responsibilities of consumers in an effort to control the price of premiums.

As part of that trend, plan designs have become increasingly complicated, with different co-pay or co-insurance levels for different kinds of services, in the plan's network of providers and outside the network, and for different types of drugs.

But even Fitzgerald, who as a McKinsey consultant worked with major health insurers to improve engagement with their customers, said she didn't expect that only 4 percent of Americans would be able to correctly define all four terms related to out-of-pocket liability.

"I wouldn't have been surprised at 10 to 20 percent," she said. "Four percent was surprising."

She noted that only 42 percent of people were able to correctly define the term out-of-pocket maximum, despite 67 percent saying the thought they definitely understood it. Ironically, the term's definition is almost literally its name: the maximum total amount a person would have to pay out of their pocket during a year for health services.

The biggest knowledge gap among consumers was when they were asked about the term co-pay, which is the fixed amount a person must pay when they visit a doctor's office, get a lab test or buy prescription drugs. Eighty-three percent of consumers said they definitely understood what a co-pay is; just 53 percent defined it correctly.

A total of 74 percent of consumers said they definitely understood the term deductible, which is the amount of money people must spend before their insurance plans begin covering the cost of health services. But only 51 percent of consumers defined that correctly.

Consumers were least confident — and with good reason — when asked to define coinsurance, which is a percentage of health costs they would be required to pay for services or medications after their deductible limit is reached. Only 47 percent of people said they definitely understand what coinsurance is; and only 22 percent actually knew what it meant when asked to define it.

When asked why she thought people had relatively low levels of knowledge about insurance terms, Fitzgerald noted studies that have found that most Americans spend a brief amount of time actually selecting and signing up for an health plan.

"It's an unpleasant topic for many people," she said. "People just don't spend that much time on it. It's intimidating, it's stressful, it's overwhelming."

"It's such a pain point for consumers."

Fitzgerald said "there's a large amount of defaulting" among people when picking insurance, going with the plan or options that their co-workers, parent or friends have selected, without giving their choices much thought.

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What's the correct name for the amount of money that a patient must pay before the insurance company begins to pay use letter keys to select choices?