If you search for 'loneliness' on Google Images you'll find that most every picture features a single person in a remote rural scene - and moreover that these people are always static, often seem to be staring, and usually have their back to the camera.
The dictionary doesn't help a great deal with answering the question. Sometimes the definitions offered relate to objective phenomena - such as when the farmhouse is isolated, or a state of being without company, etc. Sometimes they relate to the subjective phenomenon but don't tell us whether the feeling is caused by isolation or whether it is itself an apprehension of it.
Here are some rather obvious points to make about loneliness. First, that you can feel it when with others if you don't feel connected with them. You can feel it more when with others who don't understand you than when you are by yourself. Second, that when you are by yourself, even for long periods of time, you may or may not feel lonely. Solitude and loneliness are not coterminous. (Solitude can be enriching.)
A not so obvious one (that I'm just trying out). That loneliness is related to envy. That we are lonely when we feel that there is something or someone we want, and that it exists, but is not with us - but with someone else.
Or: that when we're lonely we might think that what we want is another person but that, really, what has happened is that we've located an idealised life-giving quality in the other for which we then pine. (This quality is really something which belongs, or could belong, to us ourselves.)
Another one: That the same psychological disposition which prompts loneliness also prompts irritability in close relationship. That is to say: that although the relevant feeling here, within the close relationship, is not at all loneliness, that feeling is yet a function of the fact that, here, the other is not living up to one's idealisation of them (and that we are annoyed with them for not providing something which in truth we ought to finding the courage to provide for ourselves). (The ideal never disappoints, but the person who is supposed to instantiate it can and does.)
In summary: that loneliness in isolation, and the correlative irritability in relationship, primarily involves a loss of part of the self. This, I think, ties in with the static quality of the figures in the Google image results. It's not just that these people are by themselves; it's that they are not going about any self-sustaining business (i.e. a loss of Dasein).
Naturally, our projects typically have a with-structure; our Dasein (being-in-the-world) is also a Mitsein (being-with-others). I don't mean to deny this for a moment. In fact if the self is socially constituted in this way it makes even clearer the link between loneliness and the loss of part of the self. But what I suggest here is that the best cure for much loneliness may not always be meet-ups but rather heeding a call to authenticity. (Meet-ups may after all be ways of distracting ourselves from our loneliness, rather than ways of genuinely addressing it.) The call invites us to reclaim the life-giving quality which has been projected into the other. It invites us to ask ourselves: how can I live today in a way with which I can be truly pleased? How can I do something worthwhile, something self- and project- actualising, something creative, something helpful, something satisfying? (The worst trap: putting off doing the thing which would truly give us our sense of meaning and purpose because we feel too out of sorts...)
Take the image that heads the post. Now imagine the man walking with a steady purpose, taking an interest in his surroundings, or engrossed in meaningful thought about something other than himself. Imagine that he is on-the-way-to-something. Sure, to be 'cut off', to be 'isolated', from (mutually engaging) others - these can be painful states to be in. But having the company of one's own engaged-with life-projects is not a bad place to start.