What is M250 shutter speed?

On my film Nikon FA, it has a shutter setting that is battery independent called M250 (also has Bulb). I assume it stands for mechanical 250 shutter speed but what actually is it in terms of shutter speed?
Thanks in advance!

Added (1). When you say "emergency shutter", does that mean it's bad for the camera to use that mode often?

You guessed right. Both the bulb and M250 settings on your FA (great cam btw) are purely mechanical settings. The M250 setting is a kind of emergency setting which prevents your camera from becoming an expensive, totally inoperable doorstop should your batteries run out of power.

The M250 is the same speed as the normal 1/250 s.setting, but works purely mechanical and completely bypasses the electronic shutter control circuit and integrated lightmeter of your FA, even if your batteries are ok. 1/250 s.was chosen as an intermediate speed (a compromise if you will) to allow a photographer to keep on shooting in the majority of cases under daylight conditions.

Whether this speed works for everyone is debatable. The particularly power-hungry Olympus OM-4 for instance, one of the FA's competitors, used a similar emergency speed setting, but they opted for 1/60 s. Because that was the highest synchroniztion speed of the Olympus flash system.