Reduced movement is what's reduced for you. Some women have quiet babies or have babies who, due to where they attached, just don't kick in the places that women can actually feel.
I would also say that reduced movement is anything that causes you to worry and get yourself to a hospital ASAP. I don't say this from a scaremongering point of view, just someone whose perspective has changed a whole lot in the last four weeks. Someone I know of recently miscarried within a week of her due date but only went into the hospital when she mentioned casually that she hadn't felt the baby move for 24 hours and someone insisted that she went in to get it checked. It wasn't anyone's fault; there were liver abnormalities that led to the baby's death in utero, but she'll probably always have that doubt though about whether the doctors could have done something if she'd reported the reduced movement earlier. As someone who had a few weeks previously dashed off the Maternity Assessment Centre at 9.30 when I realised I hadn't felt anything since waking up at 6.30, I know now it really is better to be safe than devastated.
What happened to that poor woman is extremely rare and there probably wasn't anything that could have been done. But the baby's kicking is a sign of good health and it's pretty much the only thing we have to go on before he/she's out in the world. Just like we'd pay attention to a baby's colour, temperament and temperature out in the world, we need to pay attention to these little signs. Again though, it's what's normal for you and whatever deviates below the norm should sound alarms. If you've got a quiet baby, then it's what's reduced for you.