I have seen "DMC" in many names of digital cameras. Is this an abbreviation? What does it mean?
DMC is just a model code used by Panasonic for their "Lumix" cameras. It doesn't have a technical meaning other than appearing as part of their internal product codes on all Lumix cameras.
You may speculate about what the letters stand for and Panasonic may even tell you what they stand for, but it's a meaning that's not technical, but just part of the way Panasonic identifies their model names. They had to choose some sequence of letters or numbers to identify their Lumix cameras, and they happened to choose the letters DMC for some reason.
This indicates that it stands for "Digital Media Camera" but that could well be post-rationalization. It could also depend on which Panasonic representative you ask.
They have other codes for other product lines, for example their televisions have "TX" in the model code.