adversary

(UK) IPA ^(key): /ˈæd.və.s(ə)ri/ or /ˈæd.və.sɛri/ or /ædˈvɜː.sə.ri/ or /ədˈvɜː.sə.ri/
(US) IPA ^(key): /ˈæd.vər.sɛri/
Audio (US) (file)

Noun

(countable) If a person is your adversary, they are your enemy.
The history teacher said: "Our country's biggest adversary in the first war was our best ally in the second. We went from killing each other to fighting side by side in twenty years."

From the day we brought them home from the pet store, our two birds were adversaries. They hated each other, and they fought with each other so much that we had to put them in separate cages.
(countable) If a person is your adversary, you are arguing with them or competing against them; they are your opponent.
My cousin loves to play computer games with me. Even though she is only six, she is a worthy adversary. She wins all the time.

Positive
adversary

When a process is adversary, it involves people who want different things, and may fight or argue over them.
The lawyer was used to divorces being adversary, but the Lebowskis were the worst couple he had ever worked for. They could not agree on anything, they screamed at each other all the time, and sometimes they started hitting each other.

Our courts use "the adversary process" to solve legal disputes. Two people argue against each other in front of a judge, who decides who is right.
When a relationship is adversary, it is unfriendly or hostile.
Those two countries have a very adversary relationship. Every week it seems like they are about to go to war over something.

When they were teenagers, those two sisters had the most adversary relationship I have ever seen. They became more and more adversary toward each other until they finally left home. However, when they became adults, their relationship got much better.