Background: According to ethical guidelines, healthcare professionals should be able to provide care that allows for the patients’ values, customs and beliefs, and the existential issues that are communicated through them. One widely discussed issue is existential loneliness. However, much of the debate dealing with existential loneliness concludes that both the phenomenon and the concept are quite vague. Aim: To clarify what constitutes existential loneliness, and to describe its lived experiences. A further aim was to provide a definition of existential loneliness that can function as a tool for identifying the phenomenon and for differentiating it from other kinds of loneliness. Method: A literature review including theoretical and empirical studies. Different search strategies were used to gather the articles included in the study. The analysis of the empirical studies had an interpretative approach. The articles were also analysed with the aim of providing a definition of existential loneliness. This was done by means of criteria such as language, uniqueness, theory and usefulness. This study is not empirical and does therefore not require an ethics review. Results: The analysis resulted in two main characteristics. The first one was perceiving oneself as inherently separated (disconnected) from others and from the universe. The second one brings out emotional aspects of EL, such as isolation, alienation, emptiness and a feeling of being abandoned. The empirical findings were divided into two categories: experiences of EL and circumstances in which EL arises. A definition of EL is also suggested, based on the two main characteristics identified. Conclusion: In order to meet patients’ needs, it is an ethical duty for healthcare staff to be able to recognise experiences of EL, that is, to communicate with the patients about these experiences in an appropriate manner. This in turn demands knowledge about existential issues and skills to deal with them.