I am playing a battlemind with flail expertise. This allows me to knock prone an opponent instead of sliding them. Is sliding specific to just powers that say slide or is push and pull included as a slide?
No, push and pull do not count as slide.
From the RC, p211:
Push & pull are not special types of slide; push, pull, and slide are all types of forced movement.
However, here are 2 feats you may want to look at: