What is subject language?

It is both a noun (SUBject) and a verb (subJECT) in English, but besides these two definitions, there are many others:

SUBject (as e.g maths, history): tema

SUBject (i.e. vassal, servant): subdito, vasallo

SUBject (of a verb): sujeto

subJECT: (as 'put down') sujetar