Well just steering wheel shake actually. Is it just wheels need balancing or anything more sinister? Car is dead straight under heavy braking - with no hands!
Wheel balancing/flat spotted tyre/wheel bearing(s)/track rod ends/steering column uj's/steering rack/suspension bushes...........could be any one of them. Make sure everything is tight first.
Well technically alright then Steve.. however the odds of knocking all the wheels out of balance in one go without knowing that you'd done it, is a little less likely than shedding a balance weight I would suggest