There are a number of different magic variable will return the hostname of a managed node (e.g. the target system).

• 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). 

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']"
}