double ko seki
This is a double ko seki. If Black takes at a White takes at b, and vice versa. Then the first player to take cannot continue locally.
Under Japanese rules, this seki, like all seki, provides no points to either player.
Double ko seki provides an unlimited number of unremovable threats, which is a reason why some people consider that superko should be applied in most rulesets. (See also Moonshine Life)