Property of a programming language that is represented textually such that it has the same structure as it’s AST/internal representation. It’s only really possible to achieve this in s-expression based languages, i.e. Lisps.

In non-homoicon languages you can still get most of the benefits with quasi-quoters like Haskell has. It allows writing [t| IO () |] or [e| putStrLn "some string" |] respectively to represent a type or expression. If you also supported robust anti-quoting and using such quasi-quotes in pattern positions, you could write metaprograms similarly to you can in lisp despite the language not really being homoiconic.