It is a verb, (past tense of fold)participle.
For something to have been folded it would need to be folded by something, whether itAn explanation of participles can be nature, a person, or itself.
I think the accurate grammatical description isfound past participle if the word is described with a verbhere.
According to: Oxford Dictionaries the past participle is
The form ofA participle is a verblexical item, typically ending in -ed in Englishderived from a verb, which is used in forming perfectthat has some of the characteristics and passive tensesfunctions of both verbs and sometimes as an adjective, e.g. looked in have you looked?, lost in lost propertyadjectives.
The adjectival form of the term participle is participial.