adjective
1
BAD : The medium score was about 6 out of 10.
GOOD : The average score was about 6 out of 10.
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medium = neither large nor small, neither tall nor short, etc: 'The waiter was of medium height and walked with a slight limp.'
average = calculated by adding a set of quantities together and then dividing this total by the number of quantities in the set: 'The average age of students entering the college this year is 19.' 'Since 1991 house prices have been falling at an average annual rate of 8%.'
2
BAD : My ability in English is about medium.
GOOD : My ability in English is about average.
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If someone's level of skill or ability is neither high nor low, it is average : 'Elizabeth shows above average intelligence for a child of three.'