Part and portion

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Part
/pɑːt/ (n.)
An amount or section which, when combined with others, makes up the whole of something. Portion
/ˈpɔːʃᵊn/ (n.)
A part of a whole.

Well, they are exact synonyms. Thereby, we should prefer part to portion because it is shorter and simpler. On this view “portion” probably exists only to suit lawyers, who have a yen, er, ceteris paribus, for words that are longer and more complicated over ones that are short and squat.

And when true synonyms are concerned, by definition all else is equal.

A “portion” feels some what more like a unitary subdivision of a greater massed whole: a portion of fries; a portion of notes, whereas a “part” is a bit of something less plainly divisible: a part of my biscuit; a part of the security package — on this view part is to portion as less is to fewer — but the JC just made it up, so I wouldn’t bet the farm on it.