即可作为可数名词也可作为不可数名词
1、作为希望或愿望时，既可数也不可数
1 [uncountable and countable] what you think or hope will happenexpectation that For some time he lived with the expectation that he was going to die.
in (the) expectation of something Anne left Germany in the expectation of seeing her family again before very long.
against/contrary to (all) expectations Against all expectations, getting up at five is actually easier in winter.
Having raised expectations (=made people more hopeful), he went on to help only a few people.
above/below expectations Profits are below expectations.
beyond (somebody's) expectations Gina has succeeded beyond our expectations.
The show exceeded all expectations (=was much better than expected).
come/live up to (somebody's) expectations His performance did not live up to our expectations
(=was not as good as we expected)
The number of people who attended fell short of expectations (=was lower than expected).
2、作为一种对某种东西的一种期望时，可数，通常使用复数形式
2 [countable usually plural] a feeling or belief about the way something should be or how someone should behaveexpectation of Women who have high expectations of marriage are often disappointed.
Some people have totally unrealistic expectations of both medical and nursing staff.
参考资料：http://www.ldoceonline.com/dictionary/expectation