There is some concern that the two terms are used almost interchangeably which further confuses the listeners.  When it comes to storm water control, the two are similar, but different.

Technically:

DETENTION: A detention basin is intended to hold water temporarily so as to limit the damage caused by something akin to a flash flood.  It is a pond or basin with a drain of limited size.  So, when there is bad runoff, the flood water flows into the basin and slowly flows out through the drain over time.  For a single bad storm, this is effective for limiting flood damage.  For a sustained storm or flood situation, it is of little benefit.

RETENTION: A retention basin has no drain.  Water trapped in the basin can only leave via evaporation or percolation into the soil.  Such basins may contain water year-round.  These are effective in closed basins since the contents never flow to the lowest point in the basin.  They are effective for longer term storage since they do not drain and worsen a flood situation.

In the vernacular:

DETENTION: Detention is when a pupil has to stay after class and write “I will not talk in class.” on the blackboard 100 times.

RETENTION: Retention is when a pupil has to repeat fourth grade.

Linguistically:

DETENTION: Detention comes from “detain” synonymous with “delay”.  The storm water flow is delayed so that the maximum flood level is reduced.

RETENTION: Retention comes from “retain” or “keep”.  The storm water flow is captured and held indefinitely.