The A lydian dominant seventh chord is a 5-note chord consisting of the notes A, C#, E, G and D#.
You can see these notes highlighted in the interactive piano chart below.
The chord itself is often abbreviated as A7#11.
Interactive piano diagram for the A lydian dominant seventh chord
Chords that are a subset of A7#11. These contain less notes but all of them are included in A7#11.