An Introduction to Ancient Greek Quotes

“We have followed the general practice in referring to the nominative form as a "case" among four other cases. However, some modern grammarians have developed an account which goes back to Aristotle and according to which the term "noun" ('onoma') should be reserved for the nominative form, which names ('onomazein') simply, with no indication of a relation to other elements in the sentence. From its base (or "upright" or "straight" -- 'orthe', 'eutheia') form and function, a noun may undergo a "fall" ('ptosis', Latin 'casus', whence English 'case') or "inclination" ('klisis', from 'klino') towards other elements within the sentence. The roster of such fallings off is called a 'declension'. Although it is convenient to include the nominative form among the "cases," we shall occasionally refer to the other four as the 'oblique' cases.”
― Alfred Mollin and Robert Williamson, An Introduction to Ancient Greek