Ionian is the name of a mode.
Ionian is the First mode of the major scale : C D E F G A B C (C Major scale)
The second mode would start from D through EFGABC and to D – called Dorian mode.
Here are the remaining mode name for the major scale.
Start from E – Phrygian
Start from F – Lydian
Start from G – Myxolydian
Start from A – Aeolian
Start from B – Locrian
So, Ionian is the first mode of the major scale.