A copay is a flat fee you pay when you get care — $30 for a doctor visit, say — set in advance, same every time. Coinsurance is a percentage split: after you have paid your deductible, you pay a share of each bill, like 20%, and insurance pays the rest. Many health plans use both, on different services.
These two words sound like twins and behave like strangers. A copay is a price tag. Coinsurance is a percentage. Which one your plan uses for a given service can change what a visit costs you by hundreds of dollars.
The copay: simple and predictable
Your plan lists flat fees for common care: maybe $25 for a regular doctor, $50 for a specialist, $10 for a generic prescription. You pay that amount at the counter, and for many plans the service is then done — no deductible, no percentage math. Copays are the part of health insurance that feels like normal shopping, which is why people like them.
Coinsurance: the percentage split
Bigger and less routine care usually runs on coinsurance instead. It only starts after you have met your yearly deductible. From there, each bill splits by percentage — 80/20 is common — until you hit your out-of-pocket maximum for the year.
Same $1,000 specialist procedure, two different plans. On a copay-style plan, you hand over the $50 specialist copay and you are done. On a coinsurance-style plan, assuming your deductible is met, you owe 20% — $200. And if your deductible is not met yet, you could owe the whole $1,000 yourself. The procedure did not change. The plan's fine print did.
How to know which you have
Every plan publishes a one or two page Summary of Benefits, and it is written in a standard format on purpose. It lists each type of care in a row and shows, in plain columns, whether you will pay a copay, coinsurance, or the full cost until the deductible is met. That single page is the most useful document your insurer will ever give you.