Sente Seki
A seki that is achieved in sente, from the point of view of a player trying to solve a life and death problem (usually either trying to save a group that looks dead, or kill a group that looks alive). This is an obviously better result than a gote seki solution to the same problem.
Actually, this is a dodgy problem...
White is urgent (Otherwise Black plays at 3 first and kills the whole lot). The marked White stones need to get two eyes (otherwise Black surrounds, kills; and then the marked Black stones are alive! i.e: not seki) - Mystery Go Player?
Bill: Note on the diagrams. Stones that frame problems or examples, such as the marked stones, are conventionally considered to be alive. This convention spares the diagram maker from the tedium of providing two eyes for every group of stones and makes the diagrams look more natural. :)
unkx80: I tacked the diagram conventions to conventions in life and death. Some things just need to be made more explicit. =)