Thanks for writing to Scienceline with a great question! Abscisic acid (abscisin or ABA for short) is a plant hormone. Plant hormones are signal molecules that regulate different aspects of plant growth. ABA got its name because it was originally thought to control leaf abscission (leaf drop in the Fall). Today, we think that ABA is a signal molecule for a variety of processes related to winter dormancy, as well as responses to water stress. For example, ABA inhibits cell division in the vascular cambium. These are the cells in plant stems through which water is carried up the plant (xylem), and sugars carried down (phloem). By slowing growth in the vascular cambium, ABA helps prepare the plant for the harsh conditions of winter. Like most signalling molecules, ABA does lots of things in the plant, and this is just one of the known functions of ABA. I hope this answers your question. Keep em coming!