Definition of Ionian mode in English:

noun

• The mode represented by the natural diatonic scale C–C (the major scale).

  ‘his lesson is on the ionian mode used in music theory for guitar and bass. Modal theory is an important aspect of understanding music theory.’

  ‘Incidentally, when you write a major scale from the tonic note up to the tonic note, you are also forming a mode, called the ionian mode!’