Fixation is when something is stuck firmly to another surface — or when you mind is stuck obsessively on one person, thing, or idea. Your parents' fixation on healthy foods means fruit for dessert every single night.
The Latin root of fixation means "to fasten or fix," and fixation is used in this literal way in technical contexts. A science experiment might involve the fixation of one chemical to another, for example. Psychoanalyst Sigmund Freud is responsible for the "obsessive interest" meaning, emphasizing childhood fixations that carry into adulthood. You might see your attachment to your mobile phone as perfectly normal, while your parents consider it an unhealthy fixation.