To aru kagaku no railgun happens basically alongside to aru majutsu no index. So, side story is more likely the appropriate term.
While the beginning of index's plot are the earliest chronologically, railgun does take place inbetween the start and finish of it as well,so it's not a prequel (since it's events come afterward) nor a sequel (since it's events end before all of index's, I believe). Side Story is the correct term.
Answers & Comments
Verified answer
To aru kagaku no railgun happens basically alongside to aru majutsu no index. So, side story is more likely the appropriate term.
While the beginning of index's plot are the earliest chronologically, railgun does take place inbetween the start and finish of it as well,so it's not a prequel (since it's events come afterward) nor a sequel (since it's events end before all of index's, I believe). Side Story is the correct term.