Definition - What does TEMPEST mean?
TEMPEST was a classified project of the U.S. government designed to research how some devices like computers throw off electromagnetic radiation (EMR) that could compromise data security. These emanations are often called compromising emanations or compromising emissions.
The term is used somewhat confusingly and incorrectly. Technically, TEMPEST is a coverword/codename but over the years has been used by some as an acronym for Telecommunications Electronics Material Protected from Emanating Spurious Transmissions.
Techopedia explains TEMPEST
Any electronic equipment can produce emanations. These need to be protected so that sensitive information is not compromised. Compromising emanations are unintentional signals that are emitted from a device that can be gathered and therefore can disclose sensitive information.
A related term used by some branches of the military is emsec, which stands for emissions security. More recently, the term TECHSEC (TECHnical SECurity) has been used in a wider context.