The "X" represents
convergence of voice and information/data within one product.
the "DA" stands for "digital assistant", as in PDA.
In the past
PDA's had no Voice and Data features in them, until
HTC was contracted to build the First XDA
Example:
A: I wanna throw that phone Away,
B: No
Just Go to XDA and Give that
little baby some Power! While You are on it, Go to XDA TV and
check That Chick out!