(in a film, television series, or other fictional work) a piece of new information that imposes a different interpretation on previously described events, typically used to facilitate a dramatic plot shift or account for an inconsistency

we're given a retcon for Wilf's absence from Donna's wedding in ‘The Runaway Bride’: he had Spanish Flu

revise (an aspect of a fictional work) retrospectively, typically by introducing a piece of new information that imposes a different interpretation on previously described events

I think fans get more upset when characters act blatantly out of established type, or when things get retconned