Nestorianism is a heresy named after its founder, Nestorius (AD 386-450). Nestorious taught that Christ had two natures (divine and human) that were not unified into one hypostasis or “hypostatic union), which was directly contradictory to the orthodox position of the church. The (approximate) opposite of Nestorianism is Monothelitism. Nestorious actually went so far as to argue that Christ had two persons (not only two natures), both divine and human.
There are some, but few, practitioners of this heresy today.