Generically a subfeed is one panel fed from an upstream panel without its own overcurrent protection (think conductors are subject to tap rules).

Other descriptions I use are:
Thru-feed - connected directly to the bus after the incoming main.
Sub-feed - connected directly to the incoming lugs

And one that is non-confroming:
Sub-feed breaker - a separate breaker connected to the bus but still mounted inside of panel.