The sensation a human being experiences of having many possible choices open to them, which they could at will decide to take.
Many people feel that this sensation's referent feels contradicted by the claim from physics that physics is deterministic. (Or, in some baroque interpretations of quantum mechanics, deterministic plus a special random "collapse" operation which is the only non-linear, non-local, non-unitary, faster-than-light, fundamentally random operation in all of physics.) If our actions are determined by physics, how could they be free?
Yudkowsky has suggested that this is one of the relatively simpler starter problems in reductionism, and aspiring reductionists should therefore make their own run at it to see how far they get into the depths of the solutions.