Per 100.
A fraction where the denominator is always 100.
Used by Roman Emperors to tax their citizens.
To get 10%, just move the decimal one spot left.
In Plain English: Percent literally means 'Per Cent' (for every 100). 50% means 50 for every 100. It's the universal language of discounts, batteries, and grades.
In The Real World: Battery life on your phone. Sales tax. Tips at a restaurant.
Computing sales tax and forgetting to ADD it to the original price.