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 a measurement of a length is 10 meters, but the true length is 10.5 meters, the absolute error is 0.5 meters. The relative error would be 0.5 divided by 10.5, which helps to understand how significant the error is in the context of the actual measurement.