What is "modelsim"

ModelSim is a multi-language HDL simulation environment by Mentor Graphics, for simulation of hardware description languages such as VHDL, Verilog and SystemC, and includes a built-in C debugger. ModelSim can be used independently, or in conjunction with Altera Quartus or Xilinx ISE. Simulation is performed using the graphical user interface (GUI), or automatically using scripts.