I have used both terms but for different situations.
Split pin for the vast majority of uses,mainly car based.
Cotter pin when used to fix bicycle pedals to the hub...usually a solid tapered pin with a machined flat and bolt( but not always) to pull it tight.
I have a box for "bits and fasteners" and there is a range of sizes of split pins in there. So far never needed to go out and buy one to fit. Set it up years ago.
