A hypervisor is that part of the software, hardware or firmware able to create and run virtual machines.
The term is often used to indicate the physical computer on which virtual machines are created and managed, but the term does not imply the presence of a dedicated machine. Examples of hypervisors dedicated to this scope are ESXi and XenServer: these hypervisors are almost exclusively intended for virtualization. Other hypervisors are Microsoft Hyper-V or Proxmox: in these two cases the virtualization part is managed by kernel modules, but the operating system is complete, so it is possible to use the machine also for other operations. Finally, there are ‘desktop’ hypervisors: Oracle VirtualBox or VMware Player: in this case the hypervisor is just an application inside the user desktop.
Talking about dedicated machines, often terms like Host and Guest are used to identify the systems that overlap: the Host is the hypervisor itself, while the hosted virtual machines are called Guest.