An expectation is a belief about what might happen in the future, like your expectation to stay close with your best friends your whole life.
The word expectation comes from the Latin word expectationem, meaning "an awaiting." If you have great expectations, you think something good will come your way, but if keep your expectations low, you won't risk being disappointed. Expectation can also describe something that is supposed to happen, like a teacher whose expectation is that everyone comes to class prepared.