Real Dictionary

proportion
A proportion of a group or an amount is a part of it.
The proportion of one kind of person or thing in a group is the number of people or things of that kind compared to the total number of people or things in the group.
countable noun: The proportion of one amount to another is the relationship between the two amounts in terms of how much there is of each thing.

proportion in American English
the comparative relation between parts, things, or elements with respect to size, amount, degree, etc.; ratio
noun: comparative relation between things or magnitudes as to size, quantity, number, etc.; ratio
transitive verb: to adjust in proper proportion or relation, as to size, quantity, etc.

proportion in British English
to adjust in relative amount, size, etc
noun: the relationship between different things or parts with respect to comparative size, number, or degree; relative magnitude or extent; ratio