pannier
Synonym discussion
Sense: noun/1
These words all hold things, but they are used in different ways. A pannier is a side-mounted carrier attached to a bicycle, motorcycle, or pack animal, and people often talk about a matching pair. A saddlebag overlaps strongly with pannier, but it usually suggests a softer bag and is especially common for bicycles, horses, and leather motorcycle bags. A side case is narrower: it normally means a hard motorcycle box with a firm shell and lock. Basket is the broad everyday word for an open container, so it does not by itself tell you that the container hangs at the side of a vehicle or animal. If you are carrying groceries on a bike, pannier is the most exact choice.
Etymology
The word came into English through French panier, meaning a bread basket. That French form goes back to Latin panarium, a container for bread, from panis meaning bread. From the idea of a carried basket, English later extended the word to side carriers on vehicles and animals, and then to the skirt supports that widened clothing at the sides.