Internal argument

The internal argument is the argument of a verb that has to be realized inside the maximal projection of that verb. Each verb may have one or more internal arguments. The argument which is closest to the verb is sometimes called the direct internal argument, while the others are called the indirect internal arguments.

Example
The argument structure of the English transitive verb open contains an external argument (Agent) and two internal arguments (Theme and Instrument) as can be inferred from the sentence Tom opened the door with his key, where the door is the direct internal argument, and with his key the indirect internal argument.

Link
Utrecht Lexicon of Linguistics

Other languages
German internes Argument