uncountable noun that refers to suitcases and bags in general, not a single item you can count
Common Mistakes
I packed my luggage in a suitcase.
I packed my suitcase.
luggage refers to all bags collectively; a suitcase is just one type of luggage, so using both together is redundant.
Synonym discussion
Sense: noun/1
Bag is the most general word for any container made of soft material that you use to carry things. A suitcase is a specific type of bag — rectangular, often hard-sided, with a handle, designed for travel. Luggage is an uncountable noun that refers to all the suitcases and bags a person takes on a trip — you never say 'a luggage' or 'two luggages'. A backpack is carried on your back with two straps and is typically used by hikers, students, or budget travellers. For example: 'She packed her suitcase for the flight' (one item), but 'Her luggage was lost at the airport' (all her bags together).
Suitcase combines 'suit' (a set of clothes, from Old French 'suite') with 'case' (a container, from Latin 'capsa'). The word first appeared in English in the late 1600s, originally referring to a small case for carrying a suit of clothes while travelling.