A flash SMS (class 0 message) is displayed immediately on the phone's screen without being stored in the inbox; support varies by handset and most gateways do not offer it.
Flash messages use a message class that tells the handset to display the text at once, on top of whatever the user is doing, and not to save it unless the user chooses to. They were used for alerts and balance notices by operators. Modern smartphones handle them inconsistently, and carriers filter them because of their use in scams.
For application senders the safe choice is a normal message with a clear first line; notification previews already show the text instantly on every modern phone.
At SMSMeteor: messages are sent as standard class-1 SMS; flash delivery is not offered.