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.