Blockchain trilemma is the 3 main problems/dilemmas commonly faced when designing a blockchain platform, namely scalability, decentralization, and security. It is difficult for any blockchain to optimize all three simultaneously.

Blockchains should ideally be scalable so that they can process high transaction volumes, decentralized so that they are safe from single-entity control, and highly secure from all kinds of intrusions and attacks. But increased scalability usually comes at the expense of decentralization and security. Similarly, an excessive focus on decentralization and security can limit scalability. This is the crux of the blockchain trilemma that blockchain platform developers face.