The phrase "single molecule" means just one tiny particle of a substance. It refers to a very small unit that cannot be divided further. Full definition
Their work, which dramatically improves upon conventional light - sheet microscopy, allows for 3 - D imaging of single molecules, live cells, and developing embryos. (aaas.org)
Once you do that, you have the potential to make sensors for single molecules. (newscientist.com)
The carbohydrates found in starches in flour, bread and potatoes as well as in sugars from fruits, sweetened beverages and desserts are all broken down into single molecules of sugar. (healthyeating.sfgate.com)