I frequently find myself needing to describe a simple Linux command that can be performed on all major shell interpreters (ksh, bash, sh), but I don't know how to indicate that. Some examples would be:
cp are all instances of what POSIX calls utilities.
Utilities may be external (an executable file in the filesystem) or built-in (internal to the shell).
From a broader perspective, you can call them POSIX / [standard] Unix [command-line] utilities.
POSIX mandates both what utilities are part of the standard Scroll down to bullet point
Utilities on the linked page.
(must be present on conformation systems) as well as what specific options they must support and behaviors they must exhibit.
Actual platforms are typically a superset of that - both in terms of offering nonstandard utilities and having standard utilities support additional, nonstandard options.
To refer to the standardized shell language syntax, you can use the term [POSIX] shell command language, though the shorthands
/bin/sh are frequently used, based on the fact that the default shell on Unix-like systems can only be expected to implement POSIX-mandated features (and nothing more).
Note that built-in utilities are a shell-specific concept (and a POSIX-compatible shell must support the options and behaviors specified in the POSIX shell-language specification).
They cannot be called (directly) when no shell is involved, such as when using
find ... -exec.
By contrast, external utilities exist independently of the shell, even though they are typically called from the shell.