diff --git a/rfcs/0137-nix-language-version.md b/rfcs/0137-nix-language-version.md
index 5c67089..de45923 100644
--- a/rfcs/0137-nix-language-version.md
+++ b/rfcs/0137-nix-language-version.md
@@ -156,6 +156,7 @@ Once the language version is accessible though, tooling can at least be adapted
    </details>
 
 1. The language version for Nix expressions is denoted in special syntax, at the beginning of a parse unit.
+   A parse unit is any text stream, e.g. a file or string.
 
    <details><summary>Arguments</summary>