I define sniper in the context of an infantry squad or platoon as quite the same as designated marksman.
* an emphasis on camouflage, concealment, cover and deception to the point that he becomes quite invisible even while shooting (including muzzle flash and dust cloud concerns)
* aimed single shots of unusually good accuracy

To me the difference between a sniper and a designated marksman is
* DM is organic to infantry small unit, sniper at most attached to it
* sniper is more extreme; more specialised weapon, more fieldcraft training
* sniper is trained as forward observer and usually better at ranging
* sniper has greater freedom of movement relative to small infantry unit.
* sniper has his own fire discipline
* sniper isn't meant to participate in assaults

So in the context of this discussion they're practically the same and I become occasionally lazy enough to not differentiate between them.

M16A1 LMG.