Hi Daz,
You've kinda answered your own question here?
The term Flatline is generally used for ECG machines in a medical environment, and is a representation of no electrical charge flowing along a pre-determined scale.
Now you say your car keeps cutting out - This is flatlining. There really isn't any mechanical definition of flatlining as the cause could be so many different things.
You'll need to expand a little on vital details, ie model,year,symptoms,possibly some pictures, any recent chances or mods? It can be quite hard to diagnose a fault when all we've got is 'my car has just cut out' etc.
If you could provide this for us it gives us a fighting chance of helping you out.