Zero-knowledge proofs. Several major cryptographic achievements have been unlocked only in recent years, facilitating both privacy and efficiency for certain setups that were only theorized before. There’s still a lot to be done for developer tooling and libraries. The foundation is solid but the ecosystem is nascent.

So far real-world applications have been mostly in the blockchain/cryptocurrency space (privacy/anonymity for Zcash and Tornado/AZTEC, Ethereum L2s, Bulletproofs in Monero, etc) but there’s so much untapped potential for other domains still.