The Casio 991ES Plus 2nd Edition is a scientific calculator that was released by Casio in the early 2000s. It is an updated version of the original Casio 991ES Plus calculator and features additional functions such as complex numbers and matrix operations. It is a popular calculator among high school and college students, particularly in the fields of math and science. The calculator has a large display and a variety of advanced math and science functions. It is also known to be durable and long lasting, making it a popular choice for students and professionals.