The community recently changed from using the term mixin for ring signatures to using ringsize. Are these terms used interchangeably, or are there some differences between these two terms?
There is a small difference. In summary, ringsize will always equal mixin + 1. So, a mixin of 2 is the same as a ringsize of 3.
Mixin refers to the number of other inputs used in a ring signature that you do not control.
Ringsize refers to the number of total inputs used in a ring signature, including ones you control.