Krayus is correct. I should add that the term "shard" originates from Ultima Online, one of the first mainstream MMO-style games. In the original creative premise behind UO, the entire world was shattered into countless copies, or "shards," each of them a representative alternate reality.
I'm actually really wonder why the term is used, since it was often used to describe separate servers, rather than a single vast server network. In STO, everyone plays in the same "universe," so simply calling it a "universe," or similar, would appear to be more accurate. But then again, I can be nit-picky (as all of us Trekkies tend to be!)