A VMware vSphere data center is the name given to the collection of physical resources, hosts, storage, and networking which power your cluster:

- name: Create a datacenter
  vmware_datacenter:
    hostname: "{{ item.ip }}"
    username: "{{ item.username }}"
    password: "{{ item.password }}"
    datacenter_name: "my_datacenter"
    state: present
  with_items: "{{ vsphere_hosts }}"

The previous example adds the hosts listed in vsphere_hosts to the my_datacenter VMware vSphere data center.