What scalability is, and what layer 2 inherits

In brief

Scalability is a network's ability to settle more transactions, faster. But the weight of the definition sits in the condition at the end:

"The main goal of scalability is to increase transaction speed (faster finality) and transaction throughput (higher number of transactions per second) without sacrificing decentralization or security."

Speed alone is not success. A system that gets faster by giving up decentralization or security has not scaled; it has turned into something else.

What a rollup does

The most discussed approach today is the rollup. Its logic is to split the work in two:

"Rollups perform transaction execution outside layer 1 and then the data is posted to layer 1 where consensus is reached."

Transactions run outside the main chain, but their data is written to the main chain, and consensus happens there.

The second half is the critical part. Because the data lands on layer 1, a rollup does not produce its own security; it inherits Ethereum's.

Why this matters

That distinction is the key to seeing that not everything called "layer 2" is the same thing. A system that posts its data to the main chain and one that does not cannot carry the same security claim.

A network describing itself as a layer 2 tells you nothing about where its data is written. That has to be checked per network.

What it measures and what it does not

High throughput is a measurement and it can be verified. On its own it measures neither decentralization nor security.

The answer to "how many transactions per second" does not answer "how many independent nodes run this". They are separate questions, and both deserve asking.