idea vs mental object

idea and mental object both are nouns.

Word	Noun	Adjective	Verb	Adverb
idea	Yes	No	No	No
mental object	Yes	No	No	No

As nouns, mental object is a hypernym of idea; that is, mental object is a word with a broader meaning than idea:

idea: the content of cognition; the main thing you are thinking about
mental object: the sum or range of what has been perceived, discovered, or learned

Other hypernyms of idea include cognitive content, content.

idea (noun)	mental object (noun)
the content of cognition; the main thing you are thinking about	the sum or range of what has been perceived, discovered, or learned
your intention; what you intend to do
an approximate calculation of quantity or degree or worth
a personal view
(music) melodic subject of a musical composition