interface

interface Named
    name: string
end

interface Player extends Named
    hp: number
end

A structural object shape, sugar over type. A known base flattens into one table type; any other base joins with &. A field takes no private or public: an interface is a shape other code sees whole.

The word is also free as a name. interface before a name on the same line declares; local interface = {} and interface.f read the Luau way.