What are people's definitions of an "L3"?

Particularly, is it an architectural classification (whether or not there's some kind of intermediary between it and L1), or a security classification (can the attacker steal? can they freeze funds? can they steal/freeze but only at high cost?)

I'd define L3s and beyond as further modular scaling solutions that leverage a layer inception-like approach to combat state bloat faced by a single rollup, and to unlock neat features like arbitrary message passing

Source:

I'd define L3s and beyond as further modular scaling solutions that leverage a layer inception-like approach to combat state bloat faced by a single rollup, and to unlock neat features like arbitrary message passing

Source: https://taiko.xyz/docs/concepts/inception-layers