A @F02366@ whose boundaries are represented by ideal parallel planes. The planar @F02366@ thickness is defined as the distance between these planes. The terms nanometre @T06345@, micrometre @T06345@ and millimetre @T06345@ refer specifically to @F02366@ thickness in the ranges \(0.1{-}100\ \text{nm}\), \(0.1{-}100\ \unicode[Times]{x3BC}\text{m}\) and \(0.1{-}100\ \text{mm}\), respectively.