Back flap of an envelope

I am wiring a letter for a company and it said it should be signed at the back flap. Can anyone tell me what the back flap of an envelope is?

Solution 1:

You can see an image here.

Essentially it's the only movable part of the envelope, creating the seal between the front and back ends by covering the open portion. Signing the back is used as a (shoddy) security measure to ensure the letter hasn't been tampered with.

The actual "flap" will depend on the type of envelope, but they are referring to whatever part forces the seal of the envelope. Here's the practice of signing envelopes containing letters of recommendation explained.