If something has been done a certain way for a long time and has become generally accepted, it's a custom. Asking strangers for candy on one night a year would seem strange if it weren't an established custom.
If you have been imprisoned in a far off country when you were just trying to blend in, you might have misunderstood the local customs. Custom can be traced back to the Latin verb consuescere, meaning "to accustom," in other words "to get used to." Custom can also describe something that you order made just the way you want it. Your five-seated bicycle is very original; was it custom-made?