Data Encryption Key. DEKs are used to encrypt and decrypt message text. They are a form of data security often used in e-mail to keep messages from being read by others than the intended recipient. A common encryption program is PGP (Pretty Good Privacy).
A sentence or few sentences below a headline, a dek summarizes the article. Like many magazine-production terms, it's intentionally misspelled (instead of "deck") so that others involved in the process won't accidentally think it's real copy.