• Satan, whose name means adversary in the Hebrew language.

In computer science:

• Adversary (cryptography), malicious entity in cryptography whose aim is to prevent the users of the cryptosystem from achieving their goal.
• Adversary (online algorithm), the adversary model is used in online algorithms in order to show competitiveness of randomized algorithms.

In fiction:

• Adversary (comics), villain from the Marvel comics universe
• The Adversary, the main villain in the Fables comic book series
• "The Adversary" (DS9 episode), the season finale of Star Trek: Deep Space Nine's third season