to reduce the mark / grade given to sb in an exam, etc.; to reduce the price of sth
mark sb downBritish
to reduce the mark / grade given to sb in an exam, etc.
She was marked down because of poor grammar.
mark sth down
to reduce the price of sth
All goods have been marked down by 15%.
mark sth down
to make a note of sth for future use or action
The factory is already marked down for demolition.
Definitions from the Oxford Advanced Learner's Dictionary, 9th edition