A percentage is a way of expressing a proportion or a fraction as a whole number. A number such as "45%" ("45 percent" or "45 per cent") is actually shorthand for the fraction 45/100. In British English, percent is often written as two words (per cent), and the spelling as one word is considered by some to be "incorrect". In American English, percent is much more common, and is usually considered "correct".
As an illustration,
The symbol for percent "%" is a stylised form of the two zeros. (In computing, other names for the character include: ITUT: percent sign; mod; grapes. INTERCAL: doubleohseven.)
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For example, suppose that an interest rate is given as a percentage like 10%. Suppose this rises to 20%. This could be described as a 100% increase, measuring the increase relative to the initial value of the interest rate. However, many people say in practice "The interest rate has risen by 10%," meaning 10% of the full amount (giving 20%), though it strictly means 10% of the original 10% (giving 11%).
To counter this confusion, the expression "percentage points" is often used. So, in the previous example, "The interest rate has increased by 10 percentage points" would be an unambiguous expression that the rate is now 20%. However, more often the term "basis points" is used, one basis point being one tenth of a percentage point.
Owners of dot com stocks came to understand that even if a stock has sunk 99%, it can nevertheless still sink another 99%. Also, if a stock rises by a large percentage, you're still broke if the stock subsequently drops 100%.
See also: Permille