Breakfast steaks are beef steaks that are cut about 15 mm thick (1/16th of an inch), though you will see steaks labelled “breakfast steaks” that are up to 1 cm thick (1/2 inch).
The idea is that you would have them at breakfast, but they can also be used for other things such as steak sandwiches.
Note though that there is no legal definition, so someone’s idea of a “steak for breakfast” might be more substantial.
See also: minute steak.
Meant to be pan-fried.