Story points are not a unit of time

(an attempt to correct some common agile estimation misconceptions)
Story points are an arbitrary unit of complexity, expected effort and doubt
You can think about, update and discuss story estimates independently of team velocity
You can't directly translate an single story estimate into hours and get a useful or accurate result
Story point estimates are meaningful only by comparison to other estimates from the same team
You can compare two stories and see that one is likely to take twice as much time as the other (on average)
You can't compare one team's velocity to another independently-estimating team's and get a useful answer
Points are only intended to be accurate in aggregate, over many stories
You can combine many estimates and a team velocity to estimate total time, in weeks or months.
You can't accurately estimate large chunks of work without breaking them down to estimate the stories within