loose
is a loose move
In this common joseki, Black has just jumped one space, moving "quickly" and low along the upper side. Compared to a move at a, is loose.
is also a loose move
White could play at b, but after Black c, White would be "behind." For this reason, White should play keima at
, a loose move compared to b.
[edit]
Loose/Tight != Thin/Thick
Loose plays are not always thin, and tight plays are not always thick.
Chris Hayashida: You might want to consider using other terms, too. I've seen these plays referred to as fast.
unkx80: I think fast moves tend to be loose.