The following is one of my favourite definitions of acceptance written by Jon Kabat Zinn. It is included in a collection called Arriving at Your Own Door: 108 Lessons in Mindfulness. This one is #70.
Acceptance doesn’t by any stretch of the imagination mean passive resignation. Quite the opposite. It takes a huge amount of fortitude and motivation to accept what is - especially when you don’t like it - and then work mindfully as best you possibly can with the circumstances you find yourself in, and with the resources at your disposal, to be in wise relationship to what is, which may mean at some point acting to mitigate, heal, re-direct, or change what can be changed.