Signal process by name. Mostly used for stopping processes.

# To kill all processes which match:
pkill -9 "<process_name>"

# To kill all processes which match their full command instead of just the process name:
pkill -9 --full "<command_name>"

# To send SIGUSR1 signal to processes which match:
pkill -USR1 "<process_name>"

# To kill the main 'firefox' process to close the browser:
pkill --oldest "firefox"