C “augmented”. Besides Caug respectively C(aug) often written as C+5 or better just C+.
Augmented chords derive from a major chord by raising the 5th up a half step (1 fret on guitar) to an augmented 5th (#5). Thinking in notes, you can stack two major thirds over the root C (1): a major 3rd gives E (3) and another major 3rd on top of that gives the augmented 5th G# (#5).
Since the step from the #5 G# to the next root C again is a major 3rd, the augmented chord is completely symmetric, consisting of nothing else but major 3rds. Consequently the three chords Caug, Eaug und G#aug (C+, E+ and G#+) have exactly the same chord tones.
The augmented chord can not be found as a diatonic chord of the major or minor scale. It just appears naturally on the IIIrd scale degree of the harmonic and melodic minor scale. It often occurs in so called line clichés: chord progressions like CCaug C6 Caug, where only one tone changes: the 5th goes up a half-step on each chord, while the other chord tones stay the same.
The famous song Vienna by Billy Joel starts right off with an augmented chord. The first four chords of the intro are Bb+ Ab13 Adim7 Eb …
The + in chord names should only be reserved for the augmented 5th (#5). Therefore a C+7 chord may be written for a C7(#5), that on the other hand is mostly interpreted as C7(b13) for other reasons.
Unfortunately you can often find a + as a replacement for # accidentals. Mostly as a postfix, like e.g. 9+ instead of #9. Now, does a C7+ stand for C7(#5) or Cmaj7? And (even worse) C7+9, is it C7#9, Cmaj7(9) (short Cmaj9) or even C7(9,#5) (short C9#5)???
You can see, the + is mostly more confusing than helpful. You should actually only write it for aug. If a 7, maj7 or other extensions are part of the game, #5 should be preferred over +, like e.g. Cmaj7(#5).