What about:
* An encounter is a way of finding out what your PC really believes;
* An encounter is a way of finding out how hard your PC will struggle to prove s/he is right;
* An encounter is an opportunity for your PC to impose his/her will upon the world.
Or:
* An encounter is a threat that you have to avoid;
* An encounter is an obstacle that you have to circumvent;
* An encounter is something that will result in you being bested unless you best it.
There are lots of different things an encounter can be, depending on what sort of game is being played. (My first list I came up with through the lens of Burning Wheel, the second through the lens of classic D&D and T&T.)