differentiation; distinguishment

to differentiate; to distinguish

(chiefly in mathematics, physics) for simplicity; for the sake of simplicity

ease / simplicity / lightness, brevity

easy / simple, uncomplicated / brief, light, short

needed, necessary

need, necessity

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

necessary evil

necessity (quality or state of being necessary, unavoidable, or absolutely requisite)