Command line option --extra-vars can be used to create variables on the command line, like this.

ansible-playbook playbook.yml --extra-vars "foo=Hello bar=World"

In this scenario, you would not define the variable in your playbook.yml file, but you would use the variable in your playbook.yml file.

---
- hosts: linux
  tasks:
    - debug: msg="{{ foo }}"

In this example, since debug was used, using the ansible-playbook command to run the playbook will produce the following output.

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