Movement monitor. Alarm sounds after 20 seconds, if no movement even the slightest breathing movements is detected. I absolutely love this monitor, i got this one as a gift 4 months ago and never used it, still in packaging in box, and i still use the one i got when i had my son for my new baby! Really helps you stop worrying while your baby sleeps, instead of getting up and checking to make shure they are still breathing like most new moms do!