Layer 1
A blockchain that runs its own consensus and settles its own transactions, such as Bitcoin, Ethereum, or Solana.
A layer 1 defines its own rules, its own validator or miner set, and its own asset used to pay fees, and it is the security base that any layer built on top refers back to. Its capacity is bounded by the requirement that ordinary participants can validate everything, since raising throughput usually means raising the hardware and bandwidth needed to run a node, which reduces how many people can check the chain independently. Changing a layer 1 requires coordinated upgrades across client teams, node operators and block producers, which is slow by design. Competing layer 1 networks differ mainly in where they set these trade-offs, and the term itself only became common once layer 2 systems existed to contrast with.
In practice
Bitcoin, Ethereum, Solana and Avalanche are all layer 1 networks, each with its own validator set, fee asset, and upgrade process.
The common misunderstanding
Being a layer 1 says nothing about security or decentralization; a network with a handful of block producers is still a layer 1.