testicles

The testicles are the testes (singular testis). They are enclosed within the scrotum. Functionally, the testicles are gonads—reproductive glands.

Testicles and testes are both formal, straightforward terms.

As with all genitals (aka private parts), there are many, many euphemisms and slang terms for them.

By far, the most common slang term for testicles is balls. More crude but also common is nuts. Similar slang terms are stones and rocks. Gonads gives us the slang term nads (and the even slangier nards). There are many other terms, like bits and marbles. The Spanish loanword cojones is sometimes used. Like balls and other slang terms for testicles, it’s also used to mean boldness or courage. The word pair is often used in this way.

A more fanciful euphemism is the family jewels. An especially British word is ballocks (sometimes spelled bollocks), which is also used as a general swear word.

Some slang phrases, such as twig and berries, refer to the penis and the testicles together. Some terms refer to them collectively—one example is junk.

Yes, the words testicles and testes are interchangeable. Testes is more commonly used in formal and technical anatomical and medical contexts. Testicles is the straightforward term most commonly used in nontechnical contexts.