Loneliness
The painful, subjective sense of lacking wanted connection — unlike Isolation, which can be neutral, and Solitude, which can be chosen.
How it shows up
Loneliness can be felt in an empty apartment, in a crowded office, or within a marriage. It often involves a sense that no one really knows or sees you, and it may bring heightened sensitivity to social threat, which can make reaching out feel harder. It is very common and not a personal failing. What defines it is the subjective gap between the connection one has and the connection one wants.
How it differs from Isolation
Isolation is simply being apart from others; loneliness is the painful sense of lacking wanted connection, alone or not.
Notice
Notice the kind of connection you miss most, and whether it is about many people or one.
Where it is felt
Felt most in the chest, stomach and throat.
Based on Nummenmaa et al. (2014), bodily maps of emotions, and related research. The map shows where people report bodily sensation, on average; individual experience varies.