A neurosis is a pattern of worry, fear or fixed anxious thinking that goes beyond what the situation calls for, but that does not cut the person off from reality — they still know what is real, still go to work, still hold a conversation. Someone who cannot leave the house without checking the door lock nine times, or who is eaten by anxiety about their health with no illness there, would once have been said to have a neurosis. The plural is "neuroses" (say it new-ROH-seez), and the adjective is "neurotic". Here is the thing your dictionary may not tell you: doctors have largely stopped using this word. It was a central term in Freud's day and in psychiatry up to about 1980, but the official manuals dropped it and now name the particular problem instead — anxiety disorder, OCD, phobia. So in a modern clinic you will rarely hear it. Where it stayed alive is ordinary speech, and there it has softened into something much lighter and often affectionate or self-mocking: "my little neurosis about clean desks", "the whole family shares one big neurosis about money". Used that way it means a quirk, a fussy anxious habit — not an illness at all. Be careful, though: calling another person "neurotic" to their face is still an insult, suggesting they fuss and worry too much. It is much safer said about yourself.