disappointment means that something happens (most of the time a person did something) that you not expected to happen in a negative way. Where consternation means that there is something wrong. There is terror/alarm/shock. There can be consternation, but you are not disappointed. Because you expected it to happen.

Consternation = dismay or anxiety, usually at something unexpected 1. Much to my consternation, my computer hard drive crashed (broke) and I lost all my mp3 files. I am shocked/dismayed that my computer hard drive crashed. I may be disappointed by this, but it is not necessary that I be. 2. To everybody's consternation, the monsoon struck the island hard and caused a lot of damage. Everybody may be dismayed/anxious about these events. But it is not the same thing that everybody is disappointed about it. (It may be that a scientist, although having consternation that the monsoon struck the island, is not disappointed about the event, because now he can do some research about the effects of a monsoon.) Disappointment = sadness caused by the non-fulfillment of one's hopes or dreams. 3. Much to my disappointment, my computer hard drive crashed and I lost all my mp3 files. I was hoping to copy all my mp3 files to a CD, but now that my hard drive crashed, I can't. I am sad about this, because my hope of copying my mp3 files was unfulfilled. I am disappointed. But I may not have consternation, because my computer was very old and I had a feeling that my hard drive might crash at any moment. 4. To everybody's disappointment, the monsoon struck the island hard and caused a lot of damage. Everybody was hoping that the monsoon would not strike the island. But since it did, they have sadness that their hope was unfulfilled. They are disappointed. If the monsoon was not predicted to strike the island, than most people probably have consternation about it also. - Summary: consternation (dismay/anxiety) and disappointment (sadness over an unfulfilled hope) may be felt at the same time, or over the same event, but it is not necessary.

That word was completely foreign to me. It looks fit for writing.

I have never heard anyone use the word consternation in my life, I actually had to look it up. We would say dismay in everyday speech instead.