A nutrient solution was a type of liquid that provided all the nutrition necessary for a living creature.
In 2369, Amanda Rogers was tasked with finding the optimum nutrient solution for live vaccine bacilli to remain alive in stasis while being transported to Tagra IV by USS Enterprise-D. The experiment she was to perform involved adding a specific dose over and over, wait for it to be metabolized and repeat until saturation recording the rate of mitosis as she did so.
When Q suggested that she could finish it more quickly by calling on her Q abilities, she was reluctant at first, but eventually agreed. Unfortunately, much to the displeasure of Beverly Crusher, this rendered the experiment useless, as the mitosis rate had been inflated beyond the natural, causing the doctor to have to do it herself the correct way. (TNG: "True Q")