A nominal interest rate is the rate as it is quoted: the percentage in the loan agreement or on the savings account, before anything has been adjusted. The word has two opposites, and which one is meant depends on the conversation.
Set against compounding, the nominal rate is the yearly figure that gets split into equal periods. A 6% nominal rate paid monthly credits 0.5% a month, and because each month's interest then earns interest, the year actually yields 6.168%, the effective annual rate. Lenders build instalments from the nominal rate: in most countries a mortgage at 4.5% charges 0.375% a month on the balance still owed. Canada is the main exception, because the Interest Act has mortgage rates stated yearly or half-yearly, so the monthly rate there is derived from a half-yearly one instead of the quoted rate divided by 12.
Set against prices, the nominal rate is the money return before rising prices are taken off. The European Central Bank gives the case of a saver earning 2.5% in a year when prices rise 3%: the balance grows, yet buys a little less than it did. What is left after that adjustment is the real return, roughly the nominal rate minus the rise in prices, and exactly (1 + nominal) ÷ (1 + price rise) − 1.
Every calculator on this site takes the nominal rate as you would read it from a statement or an agreement, so you never convert it yourself. The loan calculator divides it by the number of payments a year, the compound interest calculator shows the effective rate it implies, and the retirement calculator treats your return as nominal and reports the balance in today's money beside it.