Logical qubit
An error-corrected qubit assembled from many noisy physical qubits. Logical qubits are the meaningful unit for cryptographic threat estimates, and current ratios run from hundreds to thousands of physical qubits per logical one.
In practice
Headline qubit counts describe physical qubits, which is why they are a poor proxy for cryptographic risk.
Related terms
The basic unit of quantum information, able to hold a superposition of 0 and 1 rather than one definite value. Physical qubits are noisy; useful computation needs many of them combined into error-corrected logical qubits.
Techniques that protect quantum information from noise by spreading one logical qubit across many physical ones. It is the central engineering obstacle between today's machines and a cryptographically relevant one.
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.