mart; small market or store
A katakana noun corresponding to English-style “mart,” especially common in shop names and compounds rather than as the default everyday word for a store.
I bought a drink at the mart in front of the station.
Many small shops have “mart” in their names.
コンビニ specifically means a convenience store, while マート is a more generic katakana word that can appear in many kinds of shop names without implying that meaning.
No borrowing source is specified. The katakana form corresponds to the English-style word “mart,” and is conventionally used in Japanese commercial naming.