The purpose of this paper is to offer a brief explanation and discussion of many key terms used in describing instrument accuracy. The terms included within this discussion are most commonly used for defining performance standards with primary sensing elements typically used in the measurement of flow, level, pressure and temperature instruments. Many of the terms used may apply to controllers, recorders and final control elements. However, the focus provided herein is the primary measurement device. The typical devices include transmitters (differential pressure and temperature) and flow meters (e.g. magnetic, vortex, turbine, variable area and positive displacement)