SpaceX

It stands for "maximum dynamic pressure", and is a fluid dynamics concept. It's the moment when the spacecraft is under the most stress, and therfore where certain things are most likely to fall apart.

It's caused by a combination of atmospheric density and velocity. To avoid issues, there's a rough rule of "don't accelerate too much until you're high enough that the atmosphere thins out" during launch, and "don't hit the atmosphere too fast" during reentry.

Here's a chart for the IFT-3 launch. At one minute you can see that acceleration decreased for a few seconds, to minimize the strength of max-Q.