Medical Definition of lactose
: a disaccharide sugar C12H22O11 that is present in milk, yields glucose and galactose upon hydrolysis, yields especially lactic acid upon fermentation, and is used chiefly in foods, medicines, and culture media (as for the manufacture of penicillin)—called also milk sugar
Seen and Heard
What made you want to look up lactose? Please tell us where you read or heard it (including the quote, if possible).