Non-declarative or Procedural memory or Implicit memory -- all labels for one of two types of long-term human memory (the other type is known as Declarative or Explicit memory). These terms refer to largely unconscious memory-skills that an individual has built such as specific skills (e.g. how to ride a bicycle, how to play a musical instrument). The person knows how to perform the skill but really is unable to declare exactly how it is happening or specifically what their brain and body are doing to pull it off. You may have heard individual performers (e.g., olympic swimmers) say something such as, "When I hit the water my brain and body just take over. I no longer consciously think about every movement." For most people, this type of memory tends to weaken somewhat with age.