In science and engineering, accuracy is the degree of conformity of a measured or calculated value to its actual or specified value. Compare and contrast with precision.
In a common analogy illustrating the difference between accuracy and precision, repeated measurements are compared to arrows that are fired at a target. Accuracy describes the closeness of arrows to the bullseye at the target center. Arrows that strike closer to the bullseye are considered more accurate. The closer a system's measurements to the accepted value, the more accurate the system is considered to be.
To continue the analogy, precision would be size of the arrow cluster. When all arrows are grouped tightly together, the cluster is considered precise since they all struck close to the same spot, if not necessarily near the bullseye. The measurements are reproducible, though not necessarily accurate.
Ideally a measurement device is both accurate and precise, with measurements all close to and tightly clustered around the known value.
National standards organizations maintain weights and measures by which the accuracy of scientific and industrial devices are judged.