* Any file type associated with or attached to an e-mail. [1]
* A memorandum, letter, spreadsheet, or any other electronic document appended to another document or email. [2] [3]
* Files attached to mail message (or sometimes embedded into mail message). [4]
* An enclosure to a transmittal letter or an exhibit to a primary document. [5]
* Any electronic document appended to another document, typically email. [6]
* An attachment is a record or file associated with another record for the purpose of storage or transfer. There may be multiple attachments associated with a single “parent” or “master” record. The attachments and associated record may be managed and processed as a single unit. In common use, this term refers to a file (or files) associated with an e-mail for transfer and storage as a single message unit. Because in certain circumstances the context of the attachment—for example, the parent e-mail and its associated metadata—can be important, an organization should consider whether its policy should authorize or restrict the disassociation of attachments from their parent records. [7]

[1] RenewData, Glossary (10/5/2005).
[2] Fios, E-Discovery Glossary, http://discoveryresources.org/01_electronic_discovery_glossary.html
[3] RSI, Glossary.
[4] Ibis Consulting, Glossary.
[5] Legal Electronic Document Institute, Basic Principles of Automated Litigation Support (2005).
[6] Legal Electronic Document Institute, Basic Principles of Automated Litigation Support (2005).
[7] Kroll Ontrack, Glossary of Terms, http://www.krollontrack.com/glossaryterms

Attributions
- EDRM (http://edrm.net)