Table 2 Major elements of provided definitions and comparison to existing definitions

Feeling fundamentally alone

“Feeling as though you are alone in this existence” (P35, 17-year-old, female)

“Existential loneliness is the feeling of being alone within the universe that no amount of social interactions can change.” (P51, 21-year-old, female)

“[A]n everpresent feeling of aloneness experienced by human beings” ([33], p. 95)

Disconnection from the world and other people

“Feeling like you’re disconnected from the real world and everyone around you” (P22, 21-year-old, male)

“[T]he feeling of loneliness that can never be fixed by people around you due to feelings of detachment” (P151, 18-year-old, genderfluid)

“A feeling of being fundamentally separated from others and the world” ([9], p. 1315)

Not related to social isolation

“[H]aving the company of others but still feeling separated from everybody else” (P133, 18-year-old, female)

“Feeling like no one relates or understands to my internal thoughts, feelings, and worldviews - not just feeling that I can’t find friends or a partner to spend physical time with. I can feel ‘existentially lonely’ in a room full of people.” (P87, 37-year-old, female)

“A basic sense of loneliness that occurs when we, as human beings, face that we are separated and alone in the world despite having other people around” ([34], p. 1624)

A deep form of loneliness

“A deeper form of loneliness without an obvious cure” (P46, 36-year-old, female)

“Loneliness in a broader scale, especially when it comes to a larger, particularly cosmic scale” (P38, 17-year-old, male)

“An existential loneliness which […] extends far beyond ordinary social loneliness” ([20], p. 221)

Inability to be understood and fully share thoughts and feelings

“Feeling like no one relates or understands to my internal thoughts, feelings, and worldviews” (P87, 37-year-old, female)

“[T]hat feeling of knowing that you can never fully see exactly who anyone else is because you cant live in their own vivid individual world.” (P12, 20-year-old, female)

“[S]ince all humans are born into a world where perfect communication with others is impossible and only death is certain, a basic sense of loneliness emerges.” ([29], p. 1184)

Lack of meaning in life

“A lack of true meaning in life.” (P124, 19-year-old, female)

“Feeling completely alone in the face of an empty and infinite universe, with nothing meaningful to which i can cling” (P54, 23-year-old, female)

“[A]nd typically therefore experiencing negative feelings, that is, emotions or moods, such as […] meaninglessness or anguish” ([9], p. 1322)

Lack of purpose

“I would define existential loneliness as one losing sight of their purpose in life” (P144, 23-year-old, male)

“A feeling of not belonging because of a lack o[f] purpose” (P86, 59-year-old, male)

Purpose is generally not explicitly mentioned in definitions; however, it is included in measures [29] and other qualitative findings [32] on existential loneliness.

Existential isolation as a fact/thought

“[E]xistential loneliness is having the idea of ​​being alone in the world” (P139, 17-year-old, female)

“[T]he feeling of fear and sadness I get when I think about how I exist in the universe alone” (P131, 38-year-old, female)

“Existential loneliness is an intrinsic and organic reality of human life” ([21], p. 24)

Existential loneliness as a feeling

“Feeling that you are alone and will eventually die alone” (P27, 18-year-old, female)

“Feeling completely alone in the universe” (P28, 18-year-old, female)

“Existential loneliness describes feeling separate from other people and society” (Fried (2019) in Prohaska et al. ([35], p. 277)