An adversary is an enemy or opponent.

// Upon learning that its adversaries were ahead in sales, the company quickly formed a new advertising campaign.

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“... These students will not be better off if they know how to add up a grocery bill but have no clue about how to deal with an adversary from 10 years ago who suddenly appears before them. If the purpose of education is to prepare our children for the future, perhaps it is time to factor in the survival approach of learning how to forgive.” — Robert Enright, Psychology Today, 25 June 2024

If you consider an opponent as someone with whom one goes toe-to-toe, head-to-head, or even mano a mano, it may help you remember the meaning of adversary, a more formal synonym of opponent. Adversary comes from the Latin verb advertere, meaning “to turn or direct toward,” which makes sense; even if two adversaries are not literally face-to-face, their goals and aims are usually in direct conflict with (i.e., turned or directed toward) one another. The vertere part of advertere means “to turn,” and is the source of a number of English words, including some that are obvious, such as inadvertent and adverse, and others that are less so, like anniversary and vertebra.