Before revision: An inertial reference unit (IRU) is a type of inertial sensor which uses only gyroscopes to determine a moving aircraft’s or spacecraft’s change in angular direction (referred to as "delta-theta" or Δθ) over a period of time.
After revision: An inertial reference unit (IRU) is a type of inertial sensor which uses gyroscopes (electromechanical, ring laser gyro or MEMS) and accelerometers (electromechanical or MEMS) to determine a moving aircraft’s or spacecraft’s change in rotational attitude (angular orientation relative to some reference frame) and translational position (typically latitude, longitude and altitude) over a period of time.