Cryptographic agility
crypto-agility
The ability to change cryptographic algorithms, parameters or libraries without redesigning the systems that depend on them. Agility is what turns the next migration from a rewrite into a configuration change.
In practice
Negotiated algorithms, centralised crypto libraries and no hard-coded key sizes are the practical markers.
Related terms
A structured record of every algorithm, key, certificate, protocol, library and vendor dependency in an organisation. It is the mandatory first step of a post-quantum migration, because you cannot replace what you cannot enumerate.
Combining a classical algorithm with a post-quantum one so the result stays secure as long as either component holds. It hedges against both a future quantum attack and an undiscovered flaw in the newer post-quantum scheme.
A backwards-compatible consensus change where upgraded nodes enforce stricter rules and old nodes still accept the chain. A post-quantum signature scheme would most plausibly reach Bitcoin as a soft fork.