Latin for 'the governing noun' and 'the governed noun,' these two words are usually found in works of grammar to describe the relationship between two paired nouns.
In a phrase like "Ali's house," e.g., 'house' is the main 'governing' noun, the nomen regens while 'Ali' serves only to define the house (which house?). Ali is, therefore, the 'governed,' the nomen rectum. Another way of putting it is to ask which noun can stand on its own in a sentence and still make sense (or sufficient sense). "House burns down" makes sense if not quite satisfactorily. "Ali burns down" does not (unless the sentence is understood metaphorically).
In Hebrew grammar, this relationship is described in terms of the ">construct state (n. rectum) and the absolute (n. regens). In Greek the n. rectum would be the noun in the "genitive" case.
The use of Latin terms in works on grammar is quite common owing to the fact that grammatical studies—when it began in ernest in the West—were based on a Latin model.
Just so that you are disabused of any strange imaginations about the word, the adjective rectum comes from the Latin rectus for 'straight,' as a description of how they understood the relationship of words to one another (cf. the idea of "declining" nouns). Rectum as the terminal end of the intestine got its name from being straight instead of being convoluted like the rest of the tract.
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