There are a number of different magic variable will return the hostname of a managed node.

• ansible_hostname will return the hostname of a managed node from facts
• ansible_play_batch will return the hostname of a managed node, just like the Linux hostname command, in the form of an array of strings.
• ansible_play_hosts_all will return the hostname of a managed node, just like the Linux hostname command, in the form of an array of strings.
• ansible_limit will return the hostname passed to the -l or --limit command line option.
• inventory_hostname will return the hostname of a manage node as listed in your default hosts file or your own inventory file.
• inventory_hostname_short will return the short hostname of a manage node as listed in your default hosts file or your own inventory file.
• play_hosts will return the hostname of a managed node, just like the Linux hostname command, in the form of an array of strings.

Or, the lookup pipe hostname plugin can be used to return the hostname of your control node (that's your Ansible server). 

The ansible_limit magic variable will return the contents of the -l or --limit command line option. For example, let's say you invoke the following command.

ansible-playbook foo.yml --limit server1.example.com

For example, let's say you have the following playbook.

---
- hosts: all
  tasks:
    - debug:
        msg: ansible_hostname = {{ ansible_hostname }}

    - debug:
        msg: ansible_play_batch = {{ ansible_play_batch }}

    - debug:
        msg: ansible_play_hosts_all = {{ ansible_play_hosts_all }}

    - debug:
        msg: ansible_limit = {{ ansible_limit }}

    - debug:
        msg: inventory_hostname = {{ inventory_hostname }}

    - debug:
        msg: inventory_hostname_short = {{ inventory_hostname_short }}

    - debug:
        msg: play_hosts = {{ play_hosts }}

Since the ansible_hostname will return the hostname from facts, gather_facts must be true. If gather_facts is false, something like this will be returned.

TASK [debug] 
fatal: [server1.example.com]: FAILED! => {"msg": "The task includes an option with an undefined variable.
The error was: 'ansible_hostname' is undefined\n\n
The error appears to be in '/home/john.doe/testing.yml': line 18, column 7, but may\n
be elsewhere in the file depending on the exact syntax problem.\n\n
The offending line appears to be:\n\n
  tasks:\n
    - debug:\n
      ^ here\n"}

Assuming gather_facts is true, running this playbook should return something like this.

TASK [debug]
ok: [server1.example.com] => {
    "msg: "ansible_hostname = server1"
}
ok: [server2.example.com] => {
    "msg: "ansible_hostname = server2"
}
ok: [server3.example.com] => {
    "msg: "ansible_hostname = server3"
}

TASK [debug]
ok: [server1.example.com] => {
    "msg": [
        "server1.example.com",
        "server2.example.com",
        "server3.example.com"
    ]
}

TASK [debug]
ok: [server1.example.com] => {
    "msg": [
        "server1.example.com",
        "server2.example.com",
        "server3.example.com"
    ]
}

TASK [debug]
ok: [server1.example.com] => {
    "msg: "ansible_limit = server1.example.com"
}

TASK [debug]
ok: [server1.example.com] => {
    "msg: "inventory_hostname = server1.example.com"
}
ok: [server2.example.com] => {
    "msg: "inventory_hostname = server2.example.com"
}
ok: [server3.example.com] => {
    "msg: "inventory_hostname = server3.example.com"
}

TASK [debug]
ok: [server1.example.com] => {
    "msg: "inventory hostname_short = server1"
}
ok: [server2.example.com] => {
    "msg: "inventory hostname_short = server3"
}
ok: [server3.example.com] => {
    "msg: "inventory hostname_short = server3"
}

TASK [debug]
ok: [server1.example.com] => {
    "msg: "play_hosts = ['server1.example.com', 'server2.example.com', 'server3.example.com']"
}
ok: [server2.example.com] => {
    "msg: "play_hosts = ['server1.example.com', 'server2.example.com', 'server3.example.com']"
}
ok: [server3.example.com] => {
    "msg: "play_hosts = ['server1.example.com', 'server2.example.com', 'server3.example.com']"
}

If you are running version 2.5 or higher of Ansible, the fail module can be used to fail a play when the -l or --limit command line option was not used, meaing the ansible_limit magic variable is not defined, like this.

- name: "fail when the -l or --limit option is not used on the command line'
  fail:
   msg: "the -l or --limit option was not used on the command line"
  when: ansible_limit is not defined