Q-Day
Informal shorthand for the day a cryptographically relevant quantum computer becomes available. It is a planning device rather than a forecast — the useful question is whether your migration finishes before it, not when it lands.
Related terms
A hypothetical quantum computer large and stable enough to break deployed public-key cryptography — meaning millions of physical qubits supporting thousands of stable logical qubits. No such machine exists publicly today.
A planning rule stating that you have a problem if the time your data must stay secret, plus the time your migration takes, exceeds the time until a cryptographically relevant quantum computer arrives.
The practice of recording encrypted traffic today in order to decrypt it once a quantum computer becomes available. It is the one quantum risk already in progress, and it makes long-life confidential data the first migration priority.