Almost all children between four and twelve months of age tend to become attached to a particular object, such as a small handkerchief, a piece of cloth, the corner of a blanket, or even a stuffed toy, which they love to suck, clutch to themselves, and have close by when they are put to sleep or have to face difficulties (for example, being separated from their mother).
These objects, with which the child forms a deep bond, play a very important role in their developmental journey: they are reassuring, and they help the child overcome separation anxiety or the frustration felt in a moment of distress. They represent, in fact, something familiar to cling to, something that comforts and makes it possible to overcome the fear of anything new.
Winnicott, the English pediatrician and psychoanalyst who lived in the 20th century, called these objects transitional objects, because they appear during a particular period in a child's life: the transition between the fused phase, in which the child feels at one with the mother, and the phase in which they must gradually learn to accept being separate from her. In the first months of life, in fact, the child experiences themselves as part of the mother, who, if she is «good enough», devotes herself to meeting their needs and fulfilling their desires. As they grow, however, they must come to terms with the mother's objective inability to satisfy every desire. This creates a kind of gap between the child and the mother, which gradually leads them to realize that they are not part of her but are detached, separate from her, and that they must accept this objective reality, so far removed from what they had believed.
To learn how to do this, the child makes use of what is called the «transitional object»: a toy or anything else that allows them to experience the sense of security given by the mother's presence, precisely in order to cope with the difficulty of separation and her absence.
The transitional object is therefore a substitute for the mother, and at the same time an almost inseparable part of the child themselves.
An intermediate object, «not me», yet perceived as part of me, since it is something I possess.
As the child grows and becomes able to tell objectivity apart from subjectivity, the transitional object loses its function and is set aside, though not forgotten. This is why, even once the child no longer needs it for comfort, it's best that it only be thrown away once the child decides for themselves that they are ready to let go of it for good.
cf. D. Winnicott, Transitional Objects and Transitional Phenomena, 1953