A "sleep sack" is a special kind of clothing that covers your whole body, like a bag, and is used for sleeping. It's like a wearable blanket that keeps you warm and cozy while you sleep. Full definition
These are just identical with baby sleep sack except for the fact that it already has feet openings. (momwoot.com)
There are other great reasons to use the best sleep sack for babies. (earlymomentsmatter.org)
Instead of bending their arms and touching their environment or head, newborns in sleep sacks tend to have their arms extended and their hands closed — touching nothing. (megangarcia.com)