A portion is an adjective.
In linguistics, an adjective is a word that modifies a noun or noun phrase or describes its referent.
Its semantic role is to change information given by the noun.
Adjectives of the word portion include greater, considerable, major, lower, upper, small and large.
Portion means half of something.
The definition of a portion is part of something.
An example of portion is half of a sandwich.
Portion is defined as to divide into parts.
An example of to portion is cutting a sandwich in half.
For example, your 'portion size' may be two slices of bread in a sandwich for lunch, but only ½ slice of bread with a meal when you aren't very hungry.
Portion size is the amount that you actually eat.
For example, your 'portion sizes' at breakfast may be half a small tub of yogurt on your ½ cup muesli.