Is there an adjective form of equanimity?

There isn't a single, commonly accepted adjective form of "equanimity." However, you can express similar meanings with these options:

Formal:

* Equanimous: This is the most direct translation, meaning "calm and even-tempered."

* Serene: This implies calmness and tranquility.

* Composed: This suggests a controlled and collected demeanor.

* Unruffled: This describes someone who is not easily disturbed.

Informal:

* Cool as a cucumber: This idiom describes someone who remains calm under pressure.

* Level-headed: This describes someone with good judgment and a calm approach.

The best choice depends on the specific context and the nuance you want to convey.