The C sharp dominant flat ninth chord is a 5-note chord consisting of the notes C#, E#, G#, B and D.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as C#7b9.
Interactive piano diagram for the C sharp dominant flat ninth chord
Chords that are a subset of C#7b9. These contain less notes but all of them are included in C#7b9.