For me, equanimity is a mind quality that we are encouraged to experiment with not only during our yoga practice (on the mat) but also after our practice (off the mat) when we interact with the world. What is an equanimous mind?
An equanimous mind is one that values all mind/body states equally without the judgements and the labels of "good and bad", "right and wrong", etc.. Equanimity is all about the balance of opposites, vigilant re-alignment of body/mind/spirit, and letting go of the Ego and judgements.
Merriam-Webster says that equanimity is evenness of mind, especially under stress; balance.
Wikipedia says that equanimity is a state of psychological stability and composure which is undisturbed by experience of or exposure to emotions, pain or other phenomena that may cause others to lose the balance of their mind.
Play with concept and see what you come up with.