Interoperability
The ability of separate blockchains to communicate and transact — the multi-chain world's central technical challenge.
Interoperability is the capacity for distinct blockchains to exchange assets and data, so users and applications aren’t siloed within a single chain. As crypto fragmented into many L1s and L2s, connecting them became essential — via bridges, cross-chain messaging, and shared standards.
The vision is a seamless “internet of blockchains”; the reality is that every connection introduces trust assumptions and attack surface, and bridges have been the single largest source of hacks. The field is split between designs that maximize connectivity and those (like the modular and rollup-centric approaches) that try to inherit security from a shared base layer. Interoperability is as much a security-model debate as a technical feature.