General Definition: 1. Any thin object with two flat surfaces, one of which folds away from the other, such as a folder or an envelope, or any thing or any action associated with this. As a Noun: 1. A thin piece of cardboard folded into the shape of a container or cover, in which you can keep documents. English = folder. 2. A rectangular paper cover in which you send a letter or a card to someone through the post. English = envelope.