In physics and engineering, what does it mean when a letter in a formula has a dot above it?

Question

In physics and engineering, what does it mean when a letter in a formula has a dot above it? e.g. in a formula with "m" denoting mass, it might have a dot above the m as in the link below:

http://www.mishkas.com/inc/img/m.JPG

Answer 1

Usually it means the first derivative; two dots would be the second derivative etc.