Definition of a blanket sleeper from fleecesleeper.com

Blanket Sleeper

Using a blanket sleeper to keep your baby warm at night eliminates the potential hazards of using heavy blankets. One of the leading causes of infant death is SIDS, and one of the suspected causes of SIDS is overheating or suffocation. A blanket sleeper is typically worn as a substitute for a blanket over your baby’s regular pajamas.
A blanket sleeper is simply a sleeping bag with arm holes. It will typically zip up the front or snap shut. Some types of blanket sleepers will have Velcro or drawstring closures at the bottom. It is easy to get your baby in and out of a blanket sleeper, making it an attractive option for those middle of the night diaper changes when you are trying to disturb your baby as little as possible!
Blanket sleepers are often made from soft fleece materials, cotton or jersey knits. All sleepwear made for children is required to be treated with a flame retardant for safety. Heavier types of blanket sleepers are more appropriate for the colder weather, but even when it is mild out you will still want to use at least a light blanket for your baby at night, and a thin blanket sleeper can easily do the trick. Even the heavy fleece types are breathable, which helps prevent your baby from becoming overheated during the night.
     Below: a blanket sleeper can be a baby sleeping blanket with holes for arms, or it can more commonly refer to any baby sleeper including pajama-style sleepers; pink and purple sleepers by Tough Skins; gray with prints by My Body; khaki green and gray by Gerber; blue by Carter.

Proper fitting of a blanket sleeper is important. It should not be too loose (this makes it a potential suffocation risk), nor should it be too tight (this can restrict your baby’s movement or possibly cause them to become overheated). Parents may want to keep a few different styles and sizes on hand, since babies grow quickly and you may even need different sizes depending on what pajamas your baby is wearing underneath the blanket sleeper.