I presume the word preconscious was first used by Freud and his meaning was thoughts that were not currently conscious but could be easily made conscious all retrievable memory and perceptions that were not being attended to consciously would fit this description. It could be as wide as everything that is not repressed and not currently conscious. This does not seem a very useful designation.
A more modern use of preconscious is put forward by S Dehaene (director of INSERM-CEA group). It sounds very similar to Freud ‘a transient state of activity in which information is potentially accessible, yet not accessed’ – but is quite different. He has three processes: subliminal, preconscious and conscious. Subliminal processing happens when there is not enough strength in the bottom-up signal for it to travel forward, preconscious processing happens when there is a feed forward but there is not enough top-down attention to trigger the reverberation of the thalamo-cortical network, conscious processing has sufficient bottom-up and top-down strength to ignite consciousness. The differences are that Dehaene is talking about processes not partitions of information, that he is not talking about escaping repression (or repression at all), and that he is not talking about information that is not current (such as the total of episodic memory as opposed to that part which might in a particular moment be recalled).
A recent paper, Pupillometry: a Window on the Preconscious, by Laeng and others, put another definition in the mix. They are dealing with very young babies. The preconscious is not mentioned in the abstract or press releases and is not mentioned in other papers of their that I have read (I am again not able to read to this particular paper in the origninal). I have not seen mention of consciousness, unconsciousness or preconsciousness in their other publications I have read so far and so I assume that they have a meaning like ‘before a baby is old enough to be fully conscious’. They are dealing with very young infants. Because changes in pupil size do not register consciously at any age it is unlikely that they are discussing the sort of thing that Freud or Dehaene are. This bit of the abstract is fairly clear, given that pupillary responses can be easily measured in a noninvasive manner, occur from birth, and can occur in the absence of voluntary, conscious processes, they constitute a very promising tool for the study of preverbal (e.g., infants) or nonverbal participants (e.g., animals, neurological patients). The meaning of preconscious seems to be ‘prior to attainment of consciousness through development, evolution or healing’.
I am sure there are people, not familiar with these concepts, that would have thought the word meant ‘on the way to being conscious but not there yet’, with the pre as a simple and quick time function. From Dehaene we have, In a recent study of the attentional blink, we observed that up to about 180 ms after stimulus presentation, the occipito-temporal event-related potentials evoked by a invisible word were large and essentially indistinguishable from those evoked by a visible word. Yet on invisible trials, the participants visibility ratings did not deviate from the lowest value, used when no word was physically present. Thus, intense occipito-temporal activation can be accompanied by a complete lack of conscious report. There needs to be a word for the part of early processing that is going to be conscious in another fraction of a second as opposed to the part that is going remain unconscious. Alas, this is not a way that the word, preconscious, is used as far as I can see.
So I will continue to avoid using the word too confusing, too many possible meanings.