i am absolutely with sospan on this. I have always understood a cotter pin to be a solid tapered pin used to lock a radial member onto a rotating shaft so as to prevent relative movement betwixt shaft and member.
Split pins do a similar job, but are intended for retention rather than fixing - they are there to stop the member coming off (oo-er), but do not fix it in a mechanical sense, as the pin should never be tight in the hole.
