It's any dead body (human or animal--from anywhere in the world or possibly beyond) that has been preserved, through artificial or accidental means.
In other words, either someone made the mummy (that's why it's artificial) or something (such as an unusual act of nature) produced the mummy naturally (that's why it's accidental). In both cases, the body, after death, does not decay and become a skeleton; the skin (or fur) is somehow preserved. Even a fossil (such as the fossil of a baby dinosaur) in which the soft tissue and organs are preserved (even though they are now stone) can be considered a mummy.
Artificial mummies were made deliberately by people in various civilizations (such as ancient Egypt); a variety of processes were sometimes used--even within the same civilization.
The problem with mummies today is that they are actually disappearing all around the world. Some are disappearing because they haven't been kept very well in museums; insect damage and/or hot and too dry conditions have caused some mummies to disintegrate. Other mummies are disappearing because many people believe it is not right to look at a dead person in a museum--no matter how old the mummy is.
It makes sense that recent mummies (including those of Native Americans, found in southwest U.S. caves, dating from 400 to 600 years ago) be banned from museum exhibit (and they are--now--but it took years for Congress to pass a law stopping this practice). You see, recent mummies can often be traced to a certain family that still exists--and why would anyone allow the body of a great great great great great grandparent to be displayed for the world to see?
On the other hand, even ancient mummies (including those from Egypt) are being removed from some museums, especially in the United States. You won't find one at the Smithsonian Institution in Washington, D.C., any more. Even in Europe, some mummies (of the Guanches, for example) have been taken off display, and it's possible that certain bog bodies may be next.
Many archaeologists and other scientists believe that museum mummies provide a valuable educational service for the public. They teach about past civilizations and societies--and they also teach about the inevitability of death.