The phrase "envelope opening" refers to the act of opening an envelope, usually to reveal its contents such as letters or documents inside. Full definition
• A handy envelope opening, wide enough for daddy's hands, which allows for the option of placing the absorbent inner between the fleece and the waterproof outer layer if so desired. (myblossombottom.com)
I made this using the super easy envelope opening, you can see the tutorial here. (loveofhome.net)
The interior of the cover is a soft microfleece, and there's a wide envelope opening in each cover. (mamanista.com)