An adversary is an enemy or someone who opposes someone else. In tennis, you stand across the net from your adversary.
Adversary is related to the word adverse, meaning "against or contrary" —so think of an adversary as someone whom you are fighting against. If you're the undefeated chess champion, you have bested every single one of your adversaries. The word adversary is from Middle English adversarie, from Latin adversarius, from adversus "against."