noun /ɛnˈkloʊʒər,ɪnˈkləʊʒə/
a) Something enclosed, i.e. inserted into a letter or similar package.
There was an enclosure with the letter — a photo. <!future citation for dropbox; move to Quotations section
b) The act of enclosing, i.e. the insertion or inclusion of an item in a letter or package.
The centerpiece of the appeal is an enclosure with the letter featuring the photograph of the face of the little boy with his black eyes...
Wikipedia foundation.