For starters, the expression "nervous breakdown" is not a clinical term.

It was popularized because people would always prefer to have a physical (nerves) problem, than a psychological or psychiatric problem, but the truth is that most of the times what people call a "nervous breakdown" is really one of the following (among others):

-Panic disorder, panic attack, bipolar disorder, anxiety, traumatic stress, post-traumatic stress, psychotic disorder, depression, schizophrenia, mood disorders.

In short, independently of the specific mental illness, a "nervous breakdown" is a system overload. In other words, if one has one of the illnesses listed, an overload or shock can make it worse and/or make it noticeable (as in the case of a schizophrenia or a mood disorder). You can say then that some conditions make people prone to a nervous breakdown.

If you are perfectly healthy, the system overload or shock can trigger and become one the listed things (such in the case of a panic disorder or post-traumatic stress). In this case the nervous breakdown itself is the beginning of a psychological problem.

So, finally we could define a "nervous breakdown" as a very strong psychological experience that shocks the person consciously or unconsciusly and shows itself immediately, progressively or eventually.

Some examples:

-Being a survivor of a military unit, having witnessed the death of your mates (eventual post-traumatic stress or immediate traumatic stress).
-Being abused as a kid (progressive mood disorder or eventual depression).
-Learning that a loved one died in a terrible accident (immediate panic attack or progressive depression).

Well, you get the idea.