Q-Day

proper noun

A conjectured future day on which quantum computing developments will make it possible to defeat the encryption systems that underpin most of the applied cryptography in online use during the late 20th and early 21st centuries, including that which underpins most of the secure communications and most cryptocurrencies to date. (The defense to prevent Q-Day from having catastrophic effects would be the adoption of post-quantum cryptography as soon as possible, even before Q-Day arrives, owing to the HNDL problem.)

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Meanings

proper noun
  1. computingA conjectured future day on which quantum computing developments will make it possible to defeat the encryption systems that underpin most of the applied cryptography in online use during the late 20th and early 21st centuries, including that which underpins most of the secure communications and most cryptocurrencies to date. (The defense to prevent Q-Day from having catastrophic effects would be the adoption of post-quantum cryptography as soon as possible, even before Q-Day arrives, owing to the HNDL problem.)

Origin

The Q stands for quantum.

Full word history & related roots →
Cite this entry
APA

Q-Day. (n.d.). In WatkinsX Dictionary. Retrieved November 3, 2026, from https://dictionary.watkinsx.com/word/Q-Day

MLA

“Q-Day.” WatkinsX Dictionary, https://dictionary.watkinsx.com/word/Q-Day. Accessed November 3, 2026.

Chicago

WatkinsX Dictionary, s.v. “Q-Day,” accessed November 3, 2026, https://dictionary.watkinsx.com/word/Q-Day.

Source: Q-Day on Wiktionary, CC BY-SA 4.0.