When I was a music student, I learned that mezzo staccato means moderately short and detached. To me, this made sense because my understanding of mezzo meant moderately and staccato meant short and detached.
For the longest time, the definition seemed quite suitable. However, it seems that in the last few years there has been debate as to what it actually means. This makes sense because the term is not just used for piano music, but for other instruments as well and how you would go about making a note mezzo staccato depends on the instrument you play.
There also might be the debate about what mezzo staccato is because different composers sometimes had slightly different ideas about mezzo staccato.
To make things easier, I think mezzo staccato having the definition of moderately short and detached, is a reasonable definition.
In reality, mezzo staccato is neither the really crisp note that is achieved by bouncing a finger on a note, nor is it legato (which means smooth and well connected), where one note is nicely blended into the next note. Mezzo staccato is somewhere between the two. Some do make the mistake of treating it like staccato, but it is not as crisp as a staccato note.
The way I think of it is that staccato is where a finger bounces on a note, whereas mezzo staccato is more like jabbing at a note, thus making it not crisp and short like staccato or even blended like legato.
Mezzo staccato can be an individual note, or it can be a group of notes. In some ways, I think it is easier to achieve a single mezzo staccato note than a group of them because when there are a few of them, you must make sure that each mezzo staccato note must be separated from the next.
I have read threads of discussion about mezzo staccato and some of them seem quite confusing. Sometimes people enjoy the debating or nitpicking of someone’s argument.
I think to put it in simple terms, mezzo staccato is moderately short and detached and it needs to sound distinctly different from both staccato and legato.