An external file included with an e-mail message.
— Computer ScienceA file sent by- email- is ‘attached’ to the email. An attachment can be a picture, a document, a program or any other type of file. You should never open an attachment if you are not sure what it is, because some- viruses- propagate as email attachments; but they can’t infect your machine if you don’t open the attachment.
— Computer Science