Alien of the week
An alien of the week is a non-human race or species that only appears in one episode of Star Trek. Many of them are either members of the Federation or live on planets within Federation territory. Such races often have exotic capabilities or technologies that could significantly affect the outcomes of other events in the series if they were remembered and used, but the attributes of these one-shot races are inevitably forgotten when the time comes.
The history of the series is full of such aliens, although a few -- like the Trill -- get picked up by other writers and developed more fully.