The blockchain trilemma is the idea that a decentralised network can’t fully maximise three properties at once - security, decentralisation, and scalability. Push on one and you usually pay for it somewhere else.
The three properties are:
- security - how hard it is for an attacker to rewrite history or break the rules,
- decentralisation - how widely the power to participate is spread, without the requirement to trust anyone else,
- scalability - how many transactions the network can handle, and how cheaply.
Block size is the usual example. Bigger or more frequent blocks let you process more transactions, but they also make running a full node more expensive. Push far enough and only a handful of well-funded operators can do it, which means fewer independent verifiers and a network that’s easier to capture. Bitcoin goes the other way on purpose: it keeps the base layer cheap to verify, accepts low throughput as the cost, and moves volume onto higher layers like the Lightning Network instead.
It’s a useful way to think about trade-offs, but don’t take it too literally. The three properties aren’t cleanly separable, and “pick two” makes it sound more like a law of physics than it is. But if you want to understand why a chain can’t just raise its transaction limit for free, it’s a good place to start.