Logical qubit
An error-corrected qubit built from many physical ones — the measure that actually means something when comparing quantum computers.
Headlines cite qubit counts, but they almost always mean physical qubits, which are too noisy to compute with for any length of time. A logical qubit spreads its information across a large number of physical ones and uses continuous error correction to keep it alive. The price is steep: estimates run from hundreds to thousands of physical qubits per logical one, depending on error rates and code choice.
This is why a machine with a thousand physical qubits is nowhere near running Shor's algorithm against RSA-2048, which needs thousands of logical ones. Demonstrating a handful of stable logical qubits is the threshold the industry is currently working towards.