In Freudian psychoanalysis, the word preconscious is applied to thoughts which are unconscious at the particular moment in question, but which are not repressed and are therefore available for recall and easily capable of becoming conscious.
'Preconscious' thoughts are thus 'unconscious' in a merely 'descriptive' sense, as opposed to a 'dynamic' one.
Classical psychoanalysis therefore permits itself to
"distinguish two kinds of unconscious -- one which is easily, under frequently occurring circumstances, transformed into something conscious, and another with which this transformation is difficult and takes place only subject to a considerable expenditure of effort or possibly never at all. [. . .] We call the unconscious which is only latent, and thus easily becomes conscious, the 'preconscious', and retain the term 'unconscious' for the other". [Freud, New Introductory Lectures on Psychoanalysis (1932)]
As explained by David Stafford-Clark,
"If consciousness is then the sum total of everything of which we are aware, pre-consciousness is the reservoir of everything we can remember, all that is accessible to voluntary recall: the storehouse of memory. This leaves the unconscious area of mental life to contain all the more primitive drives and impulses influencing our actions without our necessarily ever becoming fully aware of them, together with every important constellation of ideas or memories with a strong emotional charge, which have at one time been present in consciousness but have since been repressed so that they are no longer available to it, even through introspection or attempts at memory". [David Stafford-Clark, What Freud Really Said (1965)]
Freud's original German term for the preconscious was das Vorbewusste; the unconscious being das Unbewusste.