Yes midterm exam. Shouldn't it be: Relieved that the midterm is over.
It might be in the US. If you emailed an overseas friend, they might not get it, because it's an exam, or a midterm exam, that you're referring to. You could be talking about a midterm holiday, or you could be relieved that, since the midterm (the middle of the term) is over, most of the term is over.
There's often an ambiguity when you use an adjective to mean a noun, if you assume that everyone will know which noun you mean. More often than not, such adjectives don't travel well. For example, you might know what a mobile or a cell is, but everyone knows what a phone is.
So, this might not apply to you, since you're in the US; but it's worth considering when you are posting internationally.