As a Noun: 1. The quality of having a surface which is entirely in one colour and has no pattern, design or writing. English = plainness, simplicity. 2. The quality of being very simple in style. English = plainness, simplicity. As a Verb or Adjective: 1. To have a surface which is entirely in one colour and has no pattern, design or writing. English = (be) plain, (be) undecorated. 2. To be very simple in style. English = (be) plain, (be) simple.