The correct term would be unconsciousness, however, with time, the adjective unconscious is often used as a noun.
The term unconscious started to be used more heavily in the 19th century also thanks to the popular work of Freud. His theory, that we call psychoanalysis, postulate the existence of the unconscious. This is described as a ‘place’ where we store memories and information we don’t want to know about ourselves. In this case, Freud and the psychoanalysts after him, talk about repressed psychological material. The content of the unconscious is hidden away because it would cause too much shame or damage for us to know. So this process is view as a healthy process for survival purposes. This material cannot be known by definition, however, with the use of tool such as therapy or dream analysis, it can become accessible.
Accordingly to traditional psychoanalysis, most of our live is unconscious and to live a more fulfilling life we would need to get to know this other part of ourselves. The job of the therapist is then to make the unconscious conscious and bearable, by providing a safe place and facilitate the exploration.
Other approaches, such as Cognitive and Behavioural Therapies (CBT) traditionally ignore the unconscious because they focus on how we learn things in the here and now. However, they are also recognizing the importance of knowing what we still don’t know about our process. Otherwise CBT is reduced to a quick fix and is not effective on the long run.
During the 1960s, with the development of relational psychotherapies, the perception of the therapist as an expert on someone else’s mind changed significantly. Psychoanalysis itself is becoming more and more relational, placing greater importance on the co-creation of the therapeutic work.
The second part of the 20th century saw the development of Humanistic psychotherapies and, rather than ‘unconscious’, the term used is ‘out-of-awareness’. Humanistic psychotherapies are always relational and emphasise the importance of co-creating a therapeutic alliance. Only with a trusting relationship is possible to bring into awareness all the parts of us and live a more authentic and fulfilling life.