Ansible playbook-wide variable

The set_fact module will accomplish this if group_vars don't suit your needs.

https://docs.ansible.com/ansible/latest/collections/ansible/builtin/set_fact_module.html

This module allows setting new variables. Variables are set on a host-by-host >basis just like facts discovered by the setup module. These variables will >survive between plays during an Ansible run, but will not be saved across >executions even if you use a fact cache.

- hosts: db:web
  tasks:
  - set_fact: my_global_var='hello'

- hosts: db
  tasks:
  -shell: echo {{my_global_var}} 

- hosts: web
  tasks:
  -shell: echo {{my_global_var}} 

I prefer to keep global variables in the inventory file, where you keep the groups and names of your hosts.

For example:

my-hosts:

[all:vars]
my_global_var="hello"

[db]
db1
db2
[web]
web1
web2

Run your playbook with:

ansible-playbook -i my-hosts playbook.yml

The variable will now be defined for all hosts.

If you are using ec2.py or some other dynamic inventory, you can put the global variables in the file group_vars/all to achieve the same result.