Relative error is a measure of how accurate a measurement is compared to the true value. It is calculated by taking the absolute error, which is the difference between the measured value and the true value, and dividing it by the true value. This gives a sense of the error in relation to the size of the true value, often expressed as a percentage. For example, if the true value of an object is 10 and the measured value is 9, the absolute error is 1. The relative error would be 1 divided by 10, resulting in a relative error of 0.1 or 10%. This helps to understand the significance of the error in context.