By the current definition, a sidechain is a normal blockchain that communicates (has a two-way bridge) with another blockchain (example 1, example 2).

This definition is completely useless.

Why? Because it means that basically any relevant blockchain nowadays is a sidechain. Bitcoin is a sidechain (it has a bridge with Ethereum), Ethereum is a sidechain, BSC is a sidechain etc.

A sidechain is right now just a synonym for a blockchain. Which makes the term useless.

I propose a new definition, which would require the following to be true:

• A sidechain is a chain of blocks. (Already in the current definition)
• A sidechain has a two-way bridge with the main chain. (Already in the current definition)
• A sidechain at least partially derives its security from the main chain. (New requirement in the definition)
• A sidechain cannot exist without the main chain. (New requirement in the definition)

This would clearly distinguish the terms "blockchain" (in the current multi-chain environment) and "sidechain". Sidechain becomes a hyponym of the term blockchain.

But even more importantly, this would make a clear division between the terms "main chain" and "sidechain". It would make it clear which of the chains is a sidechain, and which one is a main chain.

Additional effects of the new definition

The new definition would also make it clear that the following types of chains are considered as sidechains:

• Commit chains (but only if they have validator staking/slashing executed on the main chain)
• Rollups
• Plasma chains
• Hybrid solutions such as Volition and zkPorter

To be honest, these are sidechains even by the current definition, because they are blockchains that have a two-way peg with the main chain.

But even in the proposed new definition, these chains would fall under the sidechain umbrella, because they either partially or fully derive security from the main chain and cannot exist without it.

In this sense, the term "sidechain" could be further broken down into two categories (based on its security dependence on the main chain):

• A fully-dependent sidechain (e.g. a rollup)
• A partially-dependent sidechain (e.g. a commit chain)

An independent chain (in terms of deriving security from the main chain) by definition cannot be a sidechain.

Why is it important to revise existing definitions?

Due to the need to scale Ethereum, our community is in the midst of debating various scaling solutions, which means terms such as "sidechain" are often used. And due to an inadequate definition, this term is often misused and misunderstood. So, if we want to have quality discussions, we need to have proper and precise definitions.

Looking forward to your views on this subject. How would you define a sidechain?