Is it a hack,when you see a Player "sliding"across the map as apposed to walking or running?
no chances are it’s lag from their end the server or sometimes even your own connection
network problems like rubberbanding, lag, latency
Most likely a lag or problems with internet connection. Another thing if it’s made on purpose or not. Playing with a lag is better, at least in MoWH. Can’t say about BF though as I haven’t played it for long time.
isn’t a hack something that should give you advantage over the oppisite player/team ?
Yes it is but proving that someone’s hacking/cheating is another story. Same goes with proving that someone is deliberately lagging to gain unfair advantage.
It’s a game feature meant to annoy you.
This basically happens when the information between the player and the server isnt sending fully, which often displays it as what you have seen - or folks can be running on the spot etc. Nothing to worry about on your part.
I always LMAO when an enemy kills me and I see him sliding while crouched all over the place! 
Just a lag type thing, seen it once or twice myself.
You could say it’s an advantage, in that I will just watch them or follow them for a bit. Totally distracting!
But nah, no hack. Just a glitch.
My experience with lags comes mainly from MoHW and believe me, it is an advantage there. Players with up to 300 ms playing on european servers without any problems. It would be normal for their game being unplayable but not here. Shooting at ghosts, no hitmarks even if you hear your bullets hitting their target, getting killed behind cover or by something like OneHitKill even though he wasted half mag etc. Won’t mention sliding or teleporting. Pair it with OP weapons and you’re invincible.