VMware ESXi is a type of hypervisor that allows multiple virtual machines to run on a single physical server. It is part of the VMware virtualization suite and is designed to efficiently manage hardware resources, enabling better utilization and flexibility in IT environments. ESXi operates directly on the server's hardware, which enhances performance and reduces overhead compared to traditional operating systems. It provides a stable platform for running various operating systems and applications, making it a popular choice for businesses looking to optimize their infrastructure and reduce costs.