Refers to the doctrine of “non-self”: that there is no unchanging, permanent self, soul, or essence in living beings
It is actually an adjective, and means specifically that the item in question has no core (asāraka),
One of the three characteristics of all existence, together with dukkha (suffering, unsatisfactoriness) and anicca (impermanence).
( sa: anātman )
Good discussion at StackExchange: