adhesive envelope; gummed envelope
Refers to an envelope with a pre-applied adhesive strip that is moistened to seal, as opposed to a self-seal or plain envelope.
I licked the adhesive on the gummed envelope and sealed it.
封筒 is a general term for any envelope, while 糊付き封筒 specifically indicates one with a gummed flap.
Compound of 糊付き (のりつき, 'with glue') + 封筒 (ふうとう, 'envelope').