Loneliness and solitude are routinely treated as the same condition. They are largely independent, and separating them explains findings that otherwise appear contradictory.
The definition is about mismatch
Loneliness is described in the research literature as a discrepancy between the connection a person wants and the connection they perceive themselves to have.
Because it is a gap rather than a quantity, it can be produced either by contact falling or by expectations rising.
Which is why someone can be lonely in a busy household and content living alone, and why measuring hours spent with others predicts loneliness only weakly.
Quality of contact carries more weight than quantity
Interactions in which a person feels understood register differently from interactions in which they are present but not known.
This is the mechanism behind loneliness within relationships and within large friendly workplaces, both of which are common and both of which are underreported.
It also explains why advice to see more people frequently misses, since adding contact of the same kind does not close the gap that generated the feeling.
The state appears to be functional
Researchers have argued that loneliness operates like hunger or thirst, an aversive signal prompting a behaviour that matters for survival.
For a species dependent on group membership, detecting weakening social bonds carries real advantage, and discomfort is an effective prompt.
Which means brief loneliness following a move or a bereavement is the signal working as designed, and it is the persistent form that constitutes a problem.
Persistent loneliness alters social perception
Prolonged loneliness is associated with increased vigilance for social threat, so ambiguous behaviour from others is read more negatively.
That reading prompts withdrawal or guardedness, which tends to produce cooler responses and confirms the original expectation.
The loop is why chronic loneliness resists straightforward solutions and why interventions addressing thinking patterns have outperformed those that simply increase opportunities to meet people.
Solitude is a separate variable
Time alone that is chosen is associated with recovery, reflection and creative work, and people vary widely in how much of it they need.
The distinguishing feature is whether the solitude is chosen and whether company is available when wanted, since imposed isolation behaves very differently.
Persistent loneliness that does not shift is associated with poorer health outcomes and is worth raising with a clinician, which is a reflection of its seriousness rather than of any personal failure.