One-indexing: starting numbering things at one instead of zero
Usually this pertains to indexing arrays: people say 'oh you gotta save the space'
But! lots of algorithms actually require a sentry that is easy to check against: linked lists need nulls, trees need sentries, KMP, Fenwicks, PSTs, the list goes on
the only time you should really use zero indexing is when you have modulos, one indexing can really mess up your math.