A @[email protected] whose boundaries are represented by ideal parallel planes. The planar @[email protected] thickness is defined as the distance between these planes. The terms nanometre @[email protected], micrometre @[email protected] and millimetre @[email protected] refer specifically to @[email protected] thickness in the ranges \(0.1-100\ \text{nm}\), \(0.1-100\ \unicode[Times]{x3BC}\text{m}\) and \(0.1-100\ \text{mm}\), respectively.